[skwasm] Port to DisplayList objects (flutter#172314)

This PR refactors the skwasm renderer to use `DisplayList` objects as
its main model objects instead of using Skia objects directly. Then, at
render time, we dispatch the display list commands to the skia surface.
This is a preparatory step for impeller on web.
* Some build rules were reworked in order to allow `DisplayList` to
compile via emscripten
* Some pieces of the display list library were further refactored to
allow us to compile it without actually building and linking the
impeller shaders. The two major classes that needed to be separated out
were `DlRuntimeEffect` and the text drawing system.
* `SkPath` and `SkImage` are still used as the main model objects in
skwasm. As of right now, `DisplayList` just thinly wraps these objects,
so this is the minimal possible change for now. I will have to refactor
this somewhat further when preparing for actual impeller adoption.
* Several special cased code paths in skwasm were removed, as they are
taken care of by `DisplayList` itself. This includes shadow drawing,
determining when to enable dithering, and determining the right clamp
value for filters.

Author: eyebrowsoffire

engine/src/build/config/compiler/BUILD.gn

@@ -716,6 +716,8 @@ if (is_win) {
       # freetype2 needs these two
       "-Wno-unused-function",
       "-Wno-unused-variable",
+
+      "-Wno-sign-conversion",
     ]
   }
 
@@ -1110,10 +1112,15 @@ config("prevent_unsafe_narrowing") {
       "-Wshorten-64-to-32",
       "-Wimplicit-int-conversion",
       "-Wsign-compare",
-      "-Wsign-conversion",
       # Avoid bugs of the form `if (size_t i = size; i >= 0; --i)` while
       # fixing types to be sign-correct.
       "-Wtautological-unsigned-zero-compare",
     ]
+
+    if (!is_wasm) {
+      # emscripten's headers themselves trigger this warning, so we can't
+      # keep this enabled on the wasm target.
+      cflags += [ "-Wsign-conversion" ]
+    }
   }
 }

engine/src/flutter/display_list/BUILD.gn

@@ -298,7 +298,6 @@ if (enable_unittests) {
     deps = [
       ":display_list",
       ":display_list_fixtures",
-      "$dart_src/runtime:libdart_jit",  # for tracing
       "//flutter/benchmarking",
       "//flutter/common/graphics",
       "//flutter/display_list/testing:display_list_surface_provider",
@@ -308,6 +307,12 @@ if (enable_unittests) {
       "//flutter/testing:skia",
       "//flutter/testing:testing_lib",
     ]
+
+    if (!is_wasm) {
+      deps += [
+        "$dart_src/runtime:libdart_jit",  # for tracing
+      ]
+    }
   }
 
   executable("display_list_benchmarks") {

engine/src/flutter/fml/BUILD.gn

@@ -33,8 +33,6 @@ source_set("fml") {
     "hash_combine.h",
     "hex_codec.cc",
     "hex_codec.h",
-    "icu_util.cc",
-    "icu_util.h",
     "log_level.h",
     "log_settings.cc",
     "log_settings.h",
@@ -101,7 +99,7 @@ source_set("fml") {
     "wakeable.h",
   ]
 
-  if (enable_backtrace) {
+  if (enable_backtrace && !is_wasm) {
     sources += [ "backtrace.cc" ]
   } else {
     sources += [ "backtrace_stub.cc" ]
@@ -113,15 +111,24 @@ source_set("fml") {
     ":string_conversion",
   ]
 
-  deps = [ "//flutter/third_party/icu" ]
+  deps = []
+  if (target_os != "wasm") {
+    deps += [ "//flutter/third_party/icu" ]
+
+    sources += [
+      "icu_util.cc",
+      "icu_util.h",
+    ]
+  }
 
   if (enable_backtrace) {
     # This abseil dependency is only used by backtrace.cc.
     deps += [ "//flutter/third_party/abseil-cpp/absl/debugging:symbolize" ]
   }
 
-  configs += [ "//flutter/third_party/icu:icu_config" ]
-
+  if (target_os != "wasm") {
+    configs += [ "//flutter/third_party/icu:icu_config" ]
+  }
   public_configs = [
     "//flutter:config",
     "//flutter/common:flutter_config",

engine/src/flutter/fml/build_config.h

@@ -50,6 +50,8 @@
 #define FML_OS_SOLARIS 1
 #elif defined(__QNXNTO__)
 #define FML_OS_QNX 1
+#elif defined(__EMSCRIPTEN__)
+#define FML_OS_EMSCRIPTEN
 #else
 #error Please add support for your platform in flutter/fml/build_config.h
 #endif
@@ -106,6 +108,9 @@
 #elif defined(__pnacl__)
 #define FML_ARCH_CPU_32_BITS 1
 #define FML_ARCH_CPU_LITTLE_ENDIAN 1
+#elif defined(__EMSCRIPTEN__)
+#define FML_ARGH_CPU_32_BITS 1
+#define FML_ARCH_CPU_LITTLE_ENDIAN 1
 #else
 #error Please add support for your architecture in flutter/fml/build_config.h
 #endif

engine/src/flutter/impeller/display_list/aiks_context.h

@@ -9,6 +9,7 @@
 
 #include "impeller/entity/contents/content_context.h"
 #include "impeller/renderer/context.h"
+
 #include "impeller/renderer/render_target.h"
 #include "impeller/typographer/typographer_context.h"
 

engine/src/flutter/lib/web_ui/lib/src/engine/layers.dart

@@ -418,7 +418,7 @@ class ImageFilterOperation implements LayerOperation {
   final ui.Offset offset;
 
   @override
-  ui.Rect mapRect(ui.Rect contentRect) => filter.filterBounds(contentRect);
+  ui.Rect mapRect(ui.Rect contentRect) => filter.filterBounds(contentRect).shift(offset);
 
   @override
   void pre(SceneCanvas canvas) {

engine/src/flutter/lib/web_ui/lib/src/engine/skwasm/skwasm_impl/canvas.dart

@@ -34,16 +34,10 @@ class SkwasmCanvas implements SceneCanvas {
     final paintHandle = (paint as SkwasmPaint).toRawPaint();
     if (bounds != null) {
       withStackScope((StackScope s) {
-        canvasSaveLayer(
-          _handle,
-          s.convertRectToNative(bounds),
-          paintHandle,
-          nullptr,
-          ui.TileMode.clamp.index,
-        );
+        canvasSaveLayer(_handle, s.convertRectToNative(bounds), paintHandle, nullptr);
       });
     } else {
-      canvasSaveLayer(_handle, nullptr, paintHandle, nullptr, ui.TileMode.clamp.index);
+      canvasSaveLayer(_handle, nullptr, paintHandle, nullptr);
     }
     paintDispose(paintHandle);
   }
@@ -60,29 +54,16 @@ class SkwasmCanvas implements SceneCanvas {
     // and instead needs it supplied to the saveLayer call itself as a
     // separate argument.
     final SkwasmImageFilter nativeFilter = SkwasmImageFilter.fromUiFilter(imageFilter);
-    final ui.TileMode? backdropTileMode = nativeFilter.backdropTileMode;
     final paintHandle = (paint as SkwasmPaint).toRawPaint(/*ui.TileMode.decal*/);
     if (bounds != null) {
       withStackScope((StackScope s) {
         nativeFilter.withRawImageFilter((nativeFilterHandle) {
-          canvasSaveLayer(
-            _handle,
-            s.convertRectToNative(bounds),
-            paintHandle,
-            nativeFilterHandle,
-            (backdropTileMode ?? ui.TileMode.mirror).index,
-          );
+          canvasSaveLayer(_handle, s.convertRectToNative(bounds), paintHandle, nativeFilterHandle);
         }, defaultBlurTileMode: ui.TileMode.mirror);
       });
     } else {
       nativeFilter.withRawImageFilter((nativeFilterHandle) {
-        canvasSaveLayer(
-          _handle,
-          nullptr,
-          paintHandle,
-          nativeFilterHandle,
-          (backdropTileMode ?? ui.TileMode.mirror).index,
-        );
+        canvasSaveLayer(_handle, nullptr, paintHandle, nativeFilterHandle);
       }, defaultBlurTileMode: ui.TileMode.mirror);
     }
     paintDispose(paintHandle);