Add clamp to sk_linear_to_srgb, reorder instructions

Improves performance for xforms toSRGB and to2Dot2.  Seems
more optimal to save clamping until the end.  That way we
don't stall the mul pipeline with a min/max.
toSRGB:  371us -> 346us
to2Dot2: 404us -> 387us

FWIW, it probably makes sense to clamp inside
sk_linear_to_srgb anyway.  If not, we should potentially
provide two versions (one that clamps and one that
doesn't).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2173803002
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review-Url: https://codereview.chromium.org/2173803002
2 files changed