Add layout(srgb_unpremul) to SkSL

For 3 and 4 channel float uniforms, this states that the data supplied
is unpremul sRGB, and transforms that data to the destination color
space (still unpremul) automatically.

Change-Id: I1b420d2fd10640963fa8e6736af3747cfc6e7d5b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/286656
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
7 files changed