Add GM to test dithering image draws

We'd like to see consistent results from drawImage and imageShader, but
that's not what happens. (At least, not on GPU. CPU converts drawImage
to imageShader, so we do get dithering in both cases there).

Bug: b/320529640
Bug: b/40043601
Change-Id: Ie90f4cf5bd27d8a4ae92ede0021294b17fb51287
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/807596
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed