Part of flutter/flutter#121176
Fixes flutter/flutter#121347

Background

Imagine the following change:

typedef struct {
  /// The size of this struct. Must be sizeof(A).
  size_t struct_size;
  int one;
+ int two;
} A;

typedef struct {
  /// The size of this struct. Must be sizeof(B).
  size_t struct_size;
  A a;
  int three;
} B;

Adding a member to struct A would change the memory layout of struct B; this breaks ABI compatibility. The structs' struct_size does not protect against this ABI break. Instead, struct B should've nested struct A by pointer:

typedef struct {
  /// The size of this struct. Must be sizeof(A).
  size_t struct_size;
  int one;
+ int two;
} A;

typedef struct {
  /// The size of this struct. Must be sizeof(B).
  size_t struct_size;
  A* a;
  int three;
} B;

This problem affects the following structs:

1. FlutterTransformation - Would break FlutterSemanticsNode here
2. FlutterRect - Would break FlutterSemanticsNode here. Also, FlutterDamage is affected by the "array of structs" problem here
3. FlutterPoint would break FlutterLayer here
4. FlutterDamage - Would break FlutterPresentInfo here

I did not include the following nested structs as adding members to them does not seem to introduce any ABI issues:

1. FlutterSize
2. FlutterUIntSize
3. FlutterRoundedRect

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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