- Fallbacks for formats that have required vertex attribute support are
  removed.
- Fallbacks are changed to ones with smaller sizes
- A bug is fixed where CopyNativeVertexData wasn't initializing the
  alpha channel appropriately, which was not exercised due to fallback
  to 32-bit float formats.

Bug: b/184163871
Change-Id: Icd9afa49d94c65545d1f3fcf521881726d64529d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2809441Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>