added skeletal animation support to GPU backend

added caching of SkVertices

Docs-Preview: https://skia.org/?cl=138596
Bug: skia:
Change-Id: Ia750f55f5f6d0de250d9e9c5619f4d1ac856f9f5
Reviewed-on: https://skia-review.googlesource.com/138596
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Ruiqi Mao <ruiqimao@google.com>
28 files changed