added skeletal animation support to GPU backend

Pulled from reverted CL: https://skia-review.googlesource.com/c/skia/+/139282

Bug: skia:
Change-Id: Iad8e834aa5bbd085b2dee679cb343c5736c57907
Reviewed-on: https://skia-review.googlesource.com/140383
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ruiqi Mao <ruiqimao@google.com>
9 files changed