There are currently 2 instantiations: fixed and floating math.
There will eventually be 6: 2 math types times 3 operators (OpOver,
OpSrc and Mask).
Change-Id: I16db8f3693abe30027ea2f822909980403a6142f
Reviewed-on: https://go-review.googlesource.com/30735
Reviewed-by: David Crawshaw <crawshaw@golang.org>
