Added a GM that demonstrates the bug. Should draw a blur with
the center masked out, and a circular blurry shape that's
roughly the inverse. On raster, this was already the result.
On GPU, the blurred/eroded layer becomes a subset with an
origin other than (0,0), and that layer was shifted.
I *think* this is the correct fix - we are including 'offset'
in the texture matrices, but that's just based on the crop
rects and adjustments from each filter. We still need to adjust
the texture coords for the subsets themselves.
Bug: chromium:905548
Change-Id: I19c936adad90311aef243a9395a270d2e015df2f
Reviewed-on: https://skia-review.googlesource.com/c/173321
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
