move vertex-mode enum into SkVertices

BUG=skia:6366

Change-Id: I3c0bf96cce6d32c9b8d12d16a772aaa6f18981aa
Reviewed-on: https://skia-review.googlesource.com/11062
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
30 files changed