Add color gamut xform helpers to GrGLSLShaderBuilder

New helper functions inject the necessary shader
function. Texture lookup functions can now insert
the gamut xform at the appropriate place, too.

As written, could be used to transform non-texture
colors (e.g. vertex colors) as well.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2180803005

Review-Url: https://codereview.chromium.org/2180803005
11 files changed