simplify fill vertex calculations

Make direct2D concrete. Simplify the math.

Change-Id: I4cfa3263cedcd53108e47cfb4f503060ea13a7d8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/314458
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed