Fix GPU text colors when color spaces are involved

Fix GPU text colors when color spaces are involved

1) Only store GrColors in GrTextUtils::Paint. We still store
   premul and unpremul versions, but this is slightly clearer.
2) GrTextUtils::Paint also needs info from the render target
   context to linearize and transform the color to dst space.

Bug: skia:6605
Change-Id: I6e12c55eafaecd2a090c82b4f56827401305bf3a
Reviewed-on: https://skia-review.googlesource.com/16486
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>