Reland "have SkConvertPixels use SkColorSpaceXformSteps"

This is a reland of 6d0e566e941b09f5f9fb9f5a95c123459409c4ad

On second thought, it's probably better to correct the
types of the swizzle functions to express their required alignment.
This is a more involved CL, but I think leaves things better off.

Original change's description:
> have SkConvertPixels use SkColorSpaceXformSteps
>
> This ought to allow the fast paths in more cases, e.g. memcpy() when
> both src and dst are the same format.  Today if we tag a dst color space
> at all, we'll think we need to fall back to the general case pipeline.
>
> Some refactoring too, but no big functional change beyond using steps.
>
> Change-Id: I8fa01025229e3b9418e7f43241a2f03628a97288
> Reviewed-on: https://skia-review.googlesource.com/155640
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Mike Klein <mtklein@google.com>

Change-Id: Ia17d93acfe88a36c4c36d29e3a0b243f91178b61
Reviewed-on: https://skia-review.googlesource.com/156241
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed