hoist the common matrix code for filling vertices

Change-Id: Ic10a5b8d7c41b17de097385a0264e5379acc9687
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340136
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
4 files changed