[skwasm] Decrease reliance on finalizers/GC (#172187)

Some changes which make Skwasm less dependent on GC cycles to free its
native resources:
* Explicitly clean up pictures clipped by the scene view
* Free native `ParagraphBuilder` when `build()` is called
* Restructure `TextStyle`, `ParagraphStyle`, `StrutStyle` and
`LineMetrics` so that they don't persistently hang on to native objects
beyond a paragraph build cycle.

This addresses flutter/flutter#170889

Author: eyebrowsoffire

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

@@ -97,6 +97,7 @@ class EngineSceneView {
     final List<LayerSlice?> slices = scene.rootLayer.slices;
     final List<ScenePicture> picturesToRender = <ScenePicture>[];
     final List<ScenePicture> originalPicturesToRender = <ScenePicture>[];
+    final List<ScenePicture> picturesToFree = <ScenePicture>[];
     for (final LayerSlice? slice in slices) {
       if (slice == null) {
         continue;
@@ -111,7 +112,9 @@ class EngineSceneView {
         picturesToRender.add(slice.picture);
       } else {
         originalPicturesToRender.add(slice.picture);
-        picturesToRender.add(pictureRenderer.clipPicture(slice.picture, clippedRect));
+        final clippedPicture = pictureRenderer.clipPicture(slice.picture, clippedRect);
+        picturesToRender.add(clippedPicture);
+        picturesToFree.add(clippedPicture);
       }
     }
     final Map<ScenePicture, DomImageBitmap> renderMap;
@@ -130,6 +133,10 @@ class EngineSceneView {
     }
     recorder?.submitTimings();
 
+    for (final p in picturesToFree) {
+      p.dispose();
+    }
+
     final List<SliceContainer?> reusableContainers = List<SliceContainer?>.from(containers);
     final List<SliceContainer> newContainers = <SliceContainer>[];
     for (final LayerSlice? slice in slices) {