9654 Begin separating macOS engine from view controller (cla: yes)

9655 Allow embedders to add callbacks for responses to platform messages from the framework. (cla: yes)

9660 ExternalViewEmbedder can CancelFrame after pre-roll (cla: yes)

9661 Raster now returns an enum rather than boolean (cla: yes)

9663 Mutators Stack refactoring (cla: yes)

9667 iOS platform view opacity (cla: yes)

9668 Refactor ColorFilter to have a native wrapper (cla: yes)

9669 Improve window documentation (cla: yes)

9672 Add FLEDartProject for macOS embedding (cla: yes)

9685 fix Picture.toImage return type check and api conform test. (cla: yes)