Rename sampleVariablesSupport() to sampleMaskSupport()

We have only implemented support for the sample mask. This CL renames
the cap to reflect that.

It also replaces sampleVariablesStencilSupport() with the more aptly
named canOnlyUseSampleMaskWithStencil(), to make it more clear that
this is a driver workaround.

Change-Id: Ibf54092c63181232fb3b94cad7ddb16e3a15e3d1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251580
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
10 files changed