[graphite] Guard texture-to-texture copy with format check

WebGPU requires texture-to-texture copies to have the same format. From
a code audit, I believe all the cases that trigger texture-to-texture
copies implicitly behave this way, but this ensures we don't try to do
something illegal and fail far later.

Bug: b/390473370
Change-Id: Ib295613c4ef8fdc8c1cacef9d8183b3e2680c422
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1176077
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>