Migrate GrMeshTest to the new bind/draw API

Also adds a drawIndexPattern() helper method to GrOpsRenderPass, and
starts testing a base index with drawIndexedInstanced.

Change-Id: I3fbb1b5a2a1012a4fcbe126c54ddbc8c17ce80b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/276739
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
4 files changed