Reland [skwasm] Scene builder optimizations for platform view placement (#55468)

This is an attempt to reland the overlay optimization for skwasm and fixing the golden diffs from the framework tests.

Original PR description:

This PR refactors the scene builder's logic in order to more aggressively merge flutter content and platform view content together. This essentially covers the case discussed in this flutter issue: flutter/flutter#149863

This optimization ensures that each picture or platform view is applied to the lowest possible slice in the scene, which avoids the proliferation of redundant slices and overlays in the scene.

Author: eyebrowsoffire

lib/web_ui/lib/src/engine/color_filter.dart

@@ -135,4 +135,7 @@ class EngineColorFilter implements SceneImageFilter, ui.ColorFilter {
         return 'ColorFilter.srgbToLinearGamma()';
     }
   }
+
+  @override
+  Matrix4? get transform => null;
 }