Introduce GrColorSpaceXform, for gamut conversion on textures

Introduce GrColorSpaceXform, for gamut conversion on textures

GrTextureAccess optionally includes an instance, computed from the src
and dst color spaces. In all common cases (no color space for either src
or dst, or same color space for both), no object is allocated.

This change is orthogonal to my attempts to get color space attached to
render targets - regardless of how we choose to do that, this will give
us the source color space at all points where we are connecting src to
dst.

There are many dangling injection points where I've been inserting
nullptr, but I have a record of all of them. Additionally, there are now
three places (the most common simple paths for bitmap/image rendering)
where things are plumbed enough that I expect to have access to the dst
color space (all marked with XFORMTODO).

In addition to getting the dst color space, I need to inject shader code
and uniform uploading for appendTextureLookup and friends.

BUG=skia:
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Review-Url: https://codereview.chromium.org/2154753003