Add capability for SkColorXform to output half floats

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2147763002
CQ_INCLUDE_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot;master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review-Url: https://codereview.chromium.org/2147763002
12 files changed