Add basic multi-view rendering support

• Design documents
  • https://flutter.dev/go/multiple-views
  • https://flutter.dev/go/multi-view-embedder-apis
  • https://flutter.dev/go/desktop-multi-view-runner-apis
  • https://flutter.dev/go/multi-view-sync-over-async
• Add multi-view dart:ui APIs
  • PointerEvents coming from the engine need to contain view id #112205
• Resolve framework's warm-up frame problem
  • Add scheduleWarmUpFrame engine#50570
  • Render the warm up frame in a proper rendering process #143290
• Re-land multi-view rasterization pipeline: Reland 5: Multiview pipeline engine#51186
  • Ignore duplicate FlutterView.render calls in the same frame #144444
  • Ensure framework post-render clean up does not regress render performance  #144445
  • Properly start rasterizing before the framework cleans up #144584
• Add multi-view rendering embedder APIs
  • send window metrics with view IDs: Multi-view View Metrics engine#46174
  • Multi-view pointer event engine#46213
• Update embedders to use new multi-view embedder APIs #144810 (rendering only)
  • Windows: [Windows] Allow adding/removing views engine#51923
  • macOS
• Add new multi-view runner APIs (as "pubternal")
  • [Windows] Add multi-view runner APIs #143767
  • [macOS] Add multi-view runner APIs #145874
• Make Windows embedder multi-view ready
  • https://flutter.dev/go/windows-multi-view-ownership-updates
  • [Windows] [Multi-window] Update the engine and view ownership model #137267
  • [Windows] Allow for multiple views #143765

Work after rendering

• Make all built-in features multi-view compatible
  • Add new multi-view embedder APIs #144806
  • Built-in plugins: text input, cursor, app lifecycle, etc.
  • Accessibility
    • SemanticsActions need to be associated with a view #112207
• Make all plugins multi-view compatible
  • Decouple plugin registration from view initialization
    • Ensure plugins do not crash if there is no view
  • Make runner APIs public
  • Ensure plugins do not use runner APIs that are incompatible with multiple views
• Bugs
  • Fix multi-window pointer event crash #146251
  • flutter screenshot should work on multiple views #135534
  • Engine should drop pointer events sent to nonexistent views
  • Make Canvas.drawShadow work on multiple views #130165
• Next phase
  • Add Dart windowing APIs
    • Support window size/placement on desktop #30736
  • Update DevTools
    • Update frame timings
  • Hide new windows until first frame to prevent black flashes
  • Allow apps to disable the implicit view #144633
    • [Multiview] Make SingletonFlutterWindow safe to use when the implicit view is disabled #131651
  • Support multiple vsyncs for windows on different monitors with different refresh rates #146249
  • "Stream" scenes to improve parallelization between UI and raster threads #145712
  • [Windows] Improve performance by removing intermediary texture & blit #146013
  • [Multiview] Flutter re-renders all views during each update #164320
• Clean ups
  • Deprecate embedder APIs that aren't multi-view compatible
  • Update embedders to block the platform threads lazily when adding/removing views #146248

Other platforms:

• The Linux embedder should support multiple views or windows #138178
• The iOS shell should support multiple views or windows #138168