Also adds an example particle system that uses uniforms, and an instance
of the particles GM that draws it.
Change-Id: I64e2514fd17c8ce615b4e13b9f82424f80b8424e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/356840
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
