Add color space support to 4f gradients

Similar to the raster pipeline stage, transform the stops into the dest
color space before interpolation.

Change-Id: I626b6ef18606fd2308d7da166ce70d05f3951e21
Reviewed-on: https://skia-review.googlesource.com/18767
Reviewed-by: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
5 files changed