SkRasterPipeline: implement SkLumaColorFilter

After getting discouraged by the non-separable xfermodes, I decided to look at filling out the color filters instead.  This one's nice and easy.

There's only 1 GM that exercises this color filter, and it's drawing noticeably lighter now in f16 and sRGB configs.  565 is unchanged.  This makes me think the diffs are due to lost precision in the previous method, which was going through the default fallback to 8888 filterSpan().

I double checked: the f16 config now draws nearly identically to the gpuf16 config.  It used to be quite different.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4183
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: Ic6feaecae5cf18493b5df89733f6a5ca362e9a75
Reviewed-on: https://skia-review.googlesource.com/4183
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed