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We've always been using the portable ColorRect32, so we don't need the
ColorRectProc plumbing.

Furthermore, ColorRect32 doesn't seem to be very important (we're only using
it in the opaque case, which our row-by-row procs already specialize for).
Remove that too.

If we find we want specialization for really narrow rects again, let's put it in
blitRect() directly.  It's pretty unlikely we're going to get platform-specific
speedup for blits to non-contiguous memory.

My local SKP comparison is +- 3%... most neutral I've ever seen.

BUG=skia:

Review URL: https://codereview.chromium.org/959873002