Eliminate mixed samples as an FBO type or AA type

From now on, sample counts always refer to the number of actual color
samples, and render targets don't have separate color and stencil
sample counts.

If mixed samples support is available when making a
"GrAAType::kCoverage" draw, then an op may attach and use a mixed
sampled stencil buffer internally. But this will all be invisible to
the client.

After this CL, we temporarily won't have a mode to use nvpr with mixed
samples. That will soon be fixed by a follow-on CL that enables nvpr
with mixed samples in the normal "gl" and "gles" configs.

Bug: skia:
Change-Id: I1cb8277f0d2d0d371f24bb9f39cd473ed5c5c83b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/221878
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
110 files changed