replace SkColorSpaceXform with readPixels()

I'd like to delete SkColorSpaceXform.  We can do most of what it did
with readPixels(), as seen here.  (For the slim area where readPixels()
isn't enough, we use skcms now.)

Test: make
Change-Id: I88721adaa8a18dc7b30ed2ea20ebec1d3ed5f995