Allow custom mesh to use positions for local coordinates.

This will make it possible for drawVertices and drawCustomMesh to share
implementation in Ganesh.

Bug: skia:12720
Change-Id: I8b84119a29b47071ef389879bb5287873629ed3e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/487978
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
7 files changed