Put check for MTLTextureUsageShaderRead in available block.

MTLTextureUsage is not available on all Metal versions, so we need to
check for that. Also made the asserts consistent in this file.

Bug: skia:9573
Change-Id: I1cbb69be9c7b59d10454dc158b0f28ab5f74bbcf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254418
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
1 file changed