[graphite] Add instanced geometry for text.

Bug: skia:13118
Change-Id: Ifb738ffdefe489b08bfccedffa3d71c59818444c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/562761
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed