Refactor BitmapCanvas, lazily allocate canvas, fix image composition bug. (flutter#15153)

* Refactor BitmapCanvas. Fix image compositing bug. Allocate canvas lazily
* Fix recording canvas test by restoring context save
* Update recording canvas test for drawColor to show multiply blend

Author: ferhatb

ci/licenses_golden/licenses_flutter

@@ -387,6 +387,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/assets.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/bitmap_canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/browser_detection.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/browser_location.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/canvas_pool.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/color_filter.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/compositor/canvas_kit_canvas.dart

lib/web_ui/dev/goldens_lock.yaml

@@ -1,2 +1,2 @@
 repository: https://github.com/flutter/goldens.git
-revision: a121ff1169a1b478274a3e34c95d0a1d2d685948
+revision: c0032eeb9f9f064234991b8b5ddc15f714a53cf5

lib/web_ui/lib/src/engine.dart

@@ -23,6 +23,7 @@ part 'engine/assets.dart';
 part 'engine/bitmap_canvas.dart';
 part 'engine/browser_detection.dart';
 part 'engine/browser_location.dart';
+part 'engine/canvas_pool.dart';
 part 'engine/color_filter.dart';
 part 'engine/compositor/canvas.dart';
 part 'engine/compositor/canvas_kit_canvas.dart';