Avoid multiplication by alpha in fragment shader when known to be 1.

Implemented for image shaders, image draws, and gradient shaders.

Reimplement GrFragmentProcessor::OverrideInput as GrOverrideInputFragmentProcessor.fp.
It allows specification of whether the replacement input color should be
a literal in the shader code or a uniform. For above use case use with literal white.

Make key in variables in fp files work for 4f colors.

Fix issue in CPP code gen from .fp where when + key vars that pushed multiple values
into the shader key only skipped the first key value when the when condition is not

Bug: skia:7722

Change-Id: Id7c865132d620e8cdea8b00f2a627103eef171ac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201985
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
17 files changed