Having to refigure out what values are going into the vectors got old
very fast. The comments don't help with verifying the values, but at
least I can tell at a glance where 2(xy - wz) goes and thus determine
the "orientation" of the matrix.
gcc and clang have rather different swizzle builtins, but both do a nice
job of optimizing the intuitive initializer swizzle (I think gcc 8(?)
didn't do such a good job thus my use of __builtin_shuffle).
This seems to be pretty close to as fast as it gets (might be able to do
better with some shuffles of the negation constants instead of loading
separate constants).
