SkColorSpaceXformer: Handle compose color filters

SkColorSpaceXformer: Handle compose color filters

We could alternatively use the makeColorSpace() approach that we
adoped for loopers and imagefilters, but this seems fine.

This should finish xform canvas work on color filters.  We have
plenty of color filters that do math on colors, but only two
that might actually hold colors.

Bug: skia:6516
Change-Id: I1aee8c35d6886fdc149d4659d1bac2edc0d32b67
Reviewed-on: https://skia-review.googlesource.com/13649
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>