[svg] Perform colorspace conversions for filter effects

A filter effect can optionally be specified to operate in either sRGB
or linearRGB, according to the SVG spec:


This CL adds any necessary conversion steps (SkColorFilters) while
constructing the filter DAG. The default filter effect color space is
linearRGB. We should now be passing the filters-gauss-* W3C tests.

Specific changes:
- Tag filter effect results with their colorspace when storing them in
  the filter context map
- Add an SkColorFolor conversion step as necessary when resolving filter
  effect inputs

Bug: skia:10841
Change-Id: Ide12698ea64c4d40f09df93a60718788809086fa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/353078
Commit-Queue: Tyler Denniston <tdenniston@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
7 files changed