sk_linear_from_srgb_math

sk_linear_from_srgb_math

Looks great (imperceptibly different) but ~10% slower on both ARMv8 and x86-64.  Probably need to hide the table-or-math logic behind Sk4f/Sk8f unless we find faster math.

I do like the new look of the pipeline stages though.  A lot clearer.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2880

CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: I44952237d56ba167445b07d4830eb8959c4d47b7
Reviewed-on: https://skia-review.googlesource.com/2880
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>