Resolves https://github.com/flutter/flutter/issues/132516.
Add `impeller::HostBuffer` wrapper to Flutter GPU.
* Allows for lazy batch uploads of sparse host data to the GPU.
* Handles platform alignment requirements.
* API returns buffer view handles that will be fed to commands.
Move the GpuContext to its new home. I added a `flutter_tester` test that just verifies an exception is gracefully thrown when Impeller isn't available.
In a later patch, I'll land a way to eagerly supply the Impeller context on the cpp side to enable testing through the playground (as mentioned in https://github.com/flutter/flutter/issues/127712).
Part of https://github.com/flutter/flutter/issues/131346
Stubs a minimal test of the FFI utilities that `dart:ui` uses, but using
public symbols exported from the engine library. If this goes well, I'll
move the stuff from `dart:ui` into here and begin landing parts of the
API with test coverage.
