Added threadgroup modifier to compute shaders

Change-Id: I3a244bcd25994217d5107de98c24c144f2dabdf0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/543076
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
30 files changed