[golang-dev] Rewriting matrices and serialization in golang.org/x/mobile/exp/f32

IIRC, crawshaw wrote those Go functions. Perhaps he can remember their
providence. I have a vague memory that some example (3-D viewer??)
program used it in the past, but I can't remember the details.

Still, I'd rather just have the one consistent order everywhere, if possible.

Note that the "exp" in "x/mobile/exp/f32" stands for experimental. Its
API is far from set in stone.

I'm open to being convinced otherwise, but my instinct is that
x/image/math/f32 is not their home. It is easier to add API later than
remove it, so I'd rather err on the side of too little API for now.
Matrix multiplication seems common enough, across a variety of
applications, to warrant being inside the x/image/math/f32 package.
The case for dedicated Perspective or LookAt methods is less obvious.

If some 3-D graphics package, or program, wants to be able to set up a
camera via LookAt, it seems plausible to have that function live
inside that package, or in package main. It's not like a f32.Mat4 has
non-exported state that only in-package code can access. If its code
must be shared, it could live inside some f32util package, rather than
having to be inside x/image/math/f32 per se.