We still need *a* copy, to satisfy SkSL that mutates the input color
parameter, but it almost never needs to be global, unless there are
calls to sample() from a function other than main.
With that tweak, we generate much better SkSL/GLSL code from every one
of our effects, avoiding the proliferation of global variables.
Bug: skia:12140
Change-Id: I5c3bd42df34a095969abcbbc60e69c653a08087a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/424299
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
04d79fc594