[graphite] Split cubic sampling of YUV images from non-cubic sampling.

Set up to match what we're doing for regular image shaders.

Bug: b/293488111
Change-Id: I8d71c80200fc304e6860c3be0f662ecaf6ea6bb6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/788458
Reviewed-by: James Godfrey-Kittle <jamesgk@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
6 files changed