Vulkan: Cleanup buffer format fallbacks - 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/+/2809441 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>