added support for sk_VertexID

Change-Id: If3a2b7527ae6805ba54860c6ca6219431e2f3f76
Reviewed-on: https://skia-review.googlesource.com/8145
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
5 files changed