Fixes flutter/flutter#129073

Changes to Embedder API

Introduced FlutterRegion to represent arbitrary region:

typedef struct {
  /// The size of this struct. Must be sizeof(FlutterRegion).
  size_t struct_size;
  /// Number of rectangles in the region.
  size_t rects_count;
  /// The rectangles that make up the region.
  FlutterRect* rects;
} FlutterRegion;

Note that this is identical to struct FlutterDamage with more generic naming. Maybe down the line we could deprecate FlutterDamage and use FlutterRegion instead.

Introduced FlutterBackingStorePresentInfo:

typedef struct {
  size_t struct_size;

  /// The area of the backing store that contains Flutter contents. Pixels
  /// outside of this area are transparent and the embedder may choose not
  /// to render them. Coordinates are in physical pixels.
  FlutterRegion* paint_region;
} FlutterBackingStorePresentInfo;

In future this struct may also contain more precise hit test region (when framework supports it) and/or information relevant to partial repaint (buffer damage, frame damage).

Added a backing_store_present_info field to FlutterLayer:

typedef struct {
  ...
  /// Extra information for the backing store that the embedder may
  /// use during presentation.
  FlutterBackingStorePresentInfo* backing_store_present_info;
} FlutterLayer;

Changes to the macOS embedder

This PR adds support for FLTEnableSurfaceDebugInfo flag in main bundle Info.plist that enables visual indicators of overlay layers.

Example of unobstructed platform views

Pre-launch Checklist

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

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