Update GrFillBoundingBoxShader to use attribs

Uses an instanced attrib instead of a uniform to access the bounding
box. This will allow for batching and fewer uniform updates.

Bug: skia:10419
Change-Id: I522b4a29835564f89294a02ee4edc7a866812164
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411976
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed