Remove instanced draws and simplify GrMesh

Change-Id: I053a72892542be96fbab261eeb7fbfac76eecf01
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/276817
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed