implement SkConvertPixels color xform with steps

Opaque destinations like 565 prove the assert
I just added can't be quite so strong.

This kills off another nemesis, append_gamut_transform().

Change-Id: I5030ce094ae0f6a34d0301f6fe060b50342c7ef4
Reviewed-on: https://skia-review.googlesource.com/147106
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed