Add BGRX support to GLTextureImageBacking

GLES2 doesn't support BGRX directly but it can be stored in GL_RGB
internal format which still works. GLImageBacking with shared memory
currently works with BGRX_8888 format so in order to remove it we need
GLTextureImageBacking to also support it. Make viz::GLSupportsFormat()
return true for BGRX_8888.

Bug: 1293509
Change-Id: I394e81e06c4851c9c626862d08780bd745d61407
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3824778
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1034115}