[graphite] Add DrawWriter type for auto merging consecutive vertex data into draws

Change-Id: I94ded218e822fe50687e1efff7d46bcc4c1f1293
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/471780
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
14 files changed