Implemented DetermineBGRAReadFormatSupport in order to allow enabling
readFormat when possible with the Vulkan backend.

While reading the spec, I realized that ANGLE only seems to care
about BGRA8888 and not the new types mentioned in the extension.
Logged a new issue (http://anglebug.com/4302) to track this.

Bug: angleproject:4294
Change-Id: I071ed89574de9b4088df2301c317d3769cb31ce6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2003237
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>