Add simple mirror tiling GM

When making changes to SkImageShader in support of skia:13752 I'm
creating slight differences to mirror tiling. This simple GM with 3x1
and 1x3 images makes it much easier to see what is happening and
debug.

Bug: skia:13752
Change-Id: Ie61173f9455bbcea078e74a4e214df22f6082f2a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/615078
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed