Rename GrMesh to GrSimpleMesh

Change-Id: Ib164ff3ec41cb798385f2feea8ad09c037dbc30d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/276837
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
49 files changed