Add GrSurfaceFillContext

This is a new base class for GrSurfaceDrawContext. It allows any
alpha-type but is restricted to non-blending fills of irects using FPs,
clears,and discards.

Bug: skia:11019

Reviewed-on: https://skia-review.googlesource.com/c/skia/+/341680
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Change-Id: I696df3617719fcd8303faa73fb44b32b3fb4f71c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/344896
34 files changed