[graphite][vello] Use R8UnormStorage feature in compute path atlas

* Declare support for "storage" capability on R8Unorm textures if
  wgpu::Feature::R8UnormStorage is supported.

* Inject the "enable chromium_internal_graphite;" directive into
  WGSL shaders when running on Dawn native.

* Export the "r8" variants of the fine rasterization stage through the
  FFI bridge.

* Re-arrange the VelloComputeStep definitions for the fine stage and
  choose the R8UnormStorage variants if the Dawn feature is available.

* Request the R8UnormStorage feature when requesting a wgpu::Device in
  GraphiteDawnWindowContext.

Bug: b/328822784
Change-Id: Id516f50ce521fd57be5f2085dd904c7666b8a359
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/837638
Commit-Queue: Arman Uguray <armansito@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
9 files changed