Fix color space handling in SkImage_Gpu::getROPixels

Fix color space handling in SkImage_Gpu::getROPixels

The dstColorSpace is a badly named parameter. It's a hint about where/how
the returned pixels are going to be used. Raster and GPU are meant to
ignore that information, codecs use it to drive our decoding heuristic.

This is a re-land of https://skia-review.googlesource.com/c/10109/

Bug: skia:
Change-Id: Iee006a8e014e028b4f0f2471d7152b6bccd72cb2
Reviewed-on: https://skia-review.googlesource.com/14404
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>