enable PNG encoding on Windows for DLL builds

Windows builds a shared library (DLL) which requires all symbols to be
resolved at link time. The serialization code references SkPngEncoder::Encode,
so we must enable PNG encoding (and zlib dependency) for Windows DLL builds.

libpng will be statically linked into skia.dll from Skia's third_party
directory, keeping the DLL self-contained.

Linux/Mac continue to use static libraries with PNG disabled.

Author: anthrotype

build_skia.py

@@ -34,17 +34,31 @@
     "skia_use_icu=false",
     "skia_use_libjpeg_turbo_encode=false",
     "skia_use_libjpeg_turbo_decode=false",
-    "skia_use_libpng_encode=false",
-    "skia_use_libpng_decode=false",
     "skia_use_libwebp_encode=false",
     "skia_use_libwebp_decode=false",
     "skia_use_piex=false",
     "skia_use_xps=false",
-    "skia_use_zlib=false",
     "skia_enable_spirv_validation=false",
     "skia_use_lua=false",
     "skia_use_wuffs=false",
 ]
+
+if sys.platform == "win32":
+    # On Windows, we build a shared library (DLL) which requires all symbols to be
+    # resolved at link time. PNG encoder is referenced by serialization code, so we
+    # must enable PNG encoding (and zlib which PNG depends on) for Windows DLL builds.
+    SKIA_BUILD_ARGS.extend([
+        "skia_use_libpng_encode=true",
+        "skia_use_libpng_decode=true",
+        "skia_use_zlib=true",
+    ])
+else:
+    SKIA_BUILD_ARGS.extend([
+        "skia_use_libpng_encode=false",
+        "skia_use_libpng_decode=false",
+        "skia_use_zlib=false",
+    ])
+
 if sys.platform != "win32":
     # On Linux, I need this flag otherwise I get undefined symbol upon importing;
     # on Windows, defining this flag creates other linker issues (SkFontMgr being