Support for color-spaces with multi-stop (texture) gradients

Texture is F16 linear, unless that's not supported. In that
case, we pack down to sRGB.

Added more test patches to the gamut GM with many stops,
to test this case. Now they render correctly.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2343253002

Review-Url: https://codereview.chromium.org/2343253002
3 files changed