Reland "Use a single OffscreenCanvas for rendering in CanvasKit (flut…

…ter#45744)" (flutter#47241)

Using a single GL context avoids several issues with managing GL context
lifecycle.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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ci/licenses_golden/licenses_flutter

@@ -2621,10 +2621,11 @@ ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/picture_recorder.da
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/platform_message.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/raster_cache.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/rasterizer.dart + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/render_canvas.dart + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/render_canvas_factory.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/renderer.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/shader.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/surface.dart + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/surface_factory.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/text.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/text_fragmenter.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/util.dart + ../../../flutter/LICENSE
@@ -5400,10 +5401,11 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/picture_recorder.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/platform_message.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/raster_cache.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/rasterizer.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/render_canvas.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/render_canvas_factory.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/renderer.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/shader.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/surface.dart
-FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/surface_factory.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/text.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/text_fragmenter.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvaskit/util.dart

lib/web_ui/lib/src/engine.dart

@@ -43,10 +43,11 @@ 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/render_canvas_factory.dart';
 export 'engine/canvaskit/renderer.dart';
 export 'engine/canvaskit/shader.dart';
 export 'engine/canvaskit/surface.dart';
-export 'engine/canvaskit/surface_factory.dart';
 export 'engine/canvaskit/text.dart';
 export 'engine/canvaskit/text_fragmenter.dart';
 export 'engine/canvaskit/util.dart';

lib/web_ui/lib/src/engine/canvaskit/canvaskit_api.dart

@@ -161,6 +161,13 @@ extension CanvasKitExtension on CanvasKit {
       DomCanvasElement canvas, SkWebGLContextOptions options) =>
         _GetWebGLContext(canvas, options).toDartDouble;
 
+  @JS('GetWebGLContext')
+  external JSNumber _GetOffscreenWebGLContext(
+      DomOffscreenCanvas canvas, SkWebGLContextOptions options);
+  double GetOffscreenWebGLContext(
+          DomOffscreenCanvas canvas, SkWebGLContextOptions options) =>
+      _GetOffscreenWebGLContext(canvas, options).toDartDouble;
+
   @JS('MakeGrContext')
   external SkGrContext _MakeGrContext(JSNumber glContext);
   SkGrContext MakeGrContext(double glContext) =>
@@ -199,6 +206,9 @@ extension CanvasKitExtension on CanvasKit {
 
   external SkSurface MakeSWCanvasSurface(DomCanvasElement canvas);
 
+  @JS('MakeSWCanvasSurface')
+  external SkSurface MakeOffscreenSWCanvasSurface(DomOffscreenCanvas canvas);
+
   /// Creates an image from decoded pixels represented as a list of bytes.
   ///
   /// The pixel data must be encoded according to the image info in [info].