added skeletal animation support to GPU backend

Pulled from reverted CL: https://skia-review.googlesource.com/c/skia/+/138596

Bug: skia:
Change-Id: Ie07f1c76bae65c219ebe93d071bb19f1a30100c2
Reviewed-on: https://skia-review.googlesource.com/139282
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Ruiqi Mao <ruiqimao@google.com>
9 files changed