transform paint color to dst colorspace

Hopefully I can start to knock off effects that need
to be converted to dst colorspace one at a time now.

My rough list is,
   - paint color               <----
   - image shader
   - sprite blitter
   - gradients?
   - mode color filter
   - lighting color filter
   - high contrast filter
   - toSRGB color filter

This change cuts the diffs between 8888 and srgb from
~540 to ~500.

I left myself a note about not being able to interpret null
to sRGB at a high level yet.  That'd cause a ton of 8888 diffs,
and I think SkColorSpaceXformCanvas diffs too.

Change-Id: Id66a63e0e92130927f267719aeccb8bbcd92973a
Reviewed-on: https://skia-review.googlesource.com/140244
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

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