Progress on gamma-correctness in the GPU backend. Fixed conversion of color and profi...

Progress on gamma-correctness in the GPU backend. Fixed conversion of color and profile type to pixel config, which makes many things "just work".

Added (color=8888|f16|srgb) option to gpu configurations, along with gpuf16, gpusrgb, and anglesrgb predefined configs. Runs the gpu backend in gamma-correct mode (with either FP16 linear or sRGB 8888 frambuffers).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1750383002

Review URL: https://codereview.chromium.org/1750383002