There are no code changes, just a re-ordering so that these files are
consistent with others in this package: OpOver, OpSrc, Mask.
Change-Id: Ib1d46a8e912dae0c760af655e919b77023688189
Reviewed-on: https://go-review.googlesource.com/30111
Reviewed-by: David Crawshaw <crawshaw@golang.org>