Implement generic draw-as-clear fallback for color and stencil

This will likely have a perf hit for GL devices that trigger the clear-as-draw fallback since
the fillrectop cannot be as optimized as the direct GLSL shader. However, since the Metal
backend now needs to perform something very similar for scissored clears, I think this
improves code maintainability and is worth it.

Bug: skia:
Change-Id: Id87513784e5892c7ff3dc988115da1d39a46d8e0
Reviewed-on: https://skia-review.googlesource.com/c/182971
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
9 files changed