Optimize color xforms when src and dst are matching

Useful when:
(1) Client does not realize src and dst match (calls color
    xform anyway).
(2) Client wants half floats, src and dst have matching
    gamuts
(3) Client wants premul (done correctly in linear space),
    src and dst have matching gamuts.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2206403003

Review-Url: https://codereview.chromium.org/2206403003
4 files changed