[web] Refactor LayerScene out of CanvasKit (flutter#174375)

Moves the `LayerSceneBuilder` code out of CanvasKit and into a
higher-level directory under `engine/`. This is in preparation for the
code to be shared between the CanvasKit and Skwasm renderers.

See flutter#172311

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Author: harryterkelsen

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

@@ -21,42 +21,33 @@ export 'engine/arena.dart';
 export 'engine/browser_detection.dart';
 export 'engine/canvaskit/canvas.dart';
 export 'engine/canvaskit/canvaskit_api.dart';
-export 'engine/canvaskit/canvaskit_canvas.dart';
 export 'engine/canvaskit/color_filter.dart';
-export 'engine/canvaskit/display_canvas_factory.dart';
-export 'engine/canvaskit/embedded_views.dart';
 export 'engine/canvaskit/fonts.dart';
 export 'engine/canvaskit/image.dart';
 export 'engine/canvaskit/image_filter.dart';
 export 'engine/canvaskit/image_wasm_codecs.dart';
 export 'engine/canvaskit/image_web_codecs.dart';
-export 'engine/canvaskit/layer.dart';
-export 'engine/canvaskit/layer_scene_builder.dart';
-export 'engine/canvaskit/layer_tree.dart';
-export 'engine/canvaskit/layer_visitor.dart';
 export 'engine/canvaskit/mask_filter.dart';
 export 'engine/canvaskit/multi_surface_rasterizer.dart';
-export 'engine/canvaskit/n_way_canvas.dart';
 export 'engine/canvaskit/native_memory.dart';
 export 'engine/canvaskit/offscreen_canvas_rasterizer.dart';
-export 'engine/canvaskit/overlay_scene_optimizer.dart';
 export 'engine/canvaskit/painting.dart';
 export 'engine/canvaskit/path.dart';
 export 'engine/canvaskit/path_metrics.dart';
 export 'engine/canvaskit/picture.dart';
 export 'engine/canvaskit/picture_recorder.dart';
-export 'engine/canvaskit/raster_cache.dart';
-export 'engine/canvaskit/rasterizer.dart';
-export 'engine/canvaskit/render_canvas.dart';
 export 'engine/canvaskit/renderer.dart';
 export 'engine/canvaskit/shader.dart';
 export 'engine/canvaskit/surface.dart';
 export 'engine/canvaskit/text.dart';
-export 'engine/canvaskit/text_fragmenter.dart';
 export 'engine/canvaskit/util.dart';
 export 'engine/canvaskit/vertices.dart';
 export 'engine/clipboard.dart';
 export 'engine/color_filter.dart';
+export 'engine/compositing/composition.dart';
+export 'engine/compositing/display_canvas_factory.dart';
+export 'engine/compositing/rasterizer.dart';
+export 'engine/compositing/render_canvas.dart';
 export 'engine/configuration.dart';
 export 'engine/display.dart';
 export 'engine/dom.dart';
@@ -77,6 +68,12 @@ export 'engine/js_interop/js_promise.dart';
 export 'engine/js_interop/js_typed_data.dart';
 export 'engine/key_map.g.dart';
 export 'engine/keyboard_binding.dart';
+export 'engine/layer/layer.dart';
+export 'engine/layer/layer_painting.dart';
+export 'engine/layer/layer_scene_builder.dart';
+export 'engine/layer/layer_tree.dart';
+export 'engine/layer/layer_visitor.dart';
+export 'engine/layer/n_way_canvas.dart';
 export 'engine/layers.dart';
 export 'engine/lazy_path.dart';
 export 'engine/mouse/context_menu.dart';
@@ -91,6 +88,7 @@ export 'engine/platform_dispatcher/app_lifecycle_state.dart';
 export 'engine/platform_dispatcher/view_focus_binding.dart';
 export 'engine/platform_views.dart';
 export 'engine/platform_views/content_manager.dart';
+export 'engine/platform_views/embedder.dart';
 export 'engine/platform_views/message_handler.dart';
 export 'engine/platform_views/slots.dart';
 export 'engine/plugins.dart';
@@ -147,6 +145,7 @@ export 'engine/text_editing/input_action.dart';
 export 'engine/text_editing/input_type.dart';
 export 'engine/text_editing/text_capitalization.dart';
 export 'engine/text_editing/text_editing.dart';
+export 'engine/text_fragmenter.dart';
 export 'engine/util.dart';
 export 'engine/validators.dart';
 export 'engine/vector_math.dart';