Add sampleVariablesStencilSupport to sksl standalone caps

TBR=ethannicholas@google.com

Change-Id: I53c2896470fd202f01f51c5aaddf3087b2cbb5ce
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251556
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
3 files changed