[geometry] Discrepancy in calculations, Firefox vs my JS implementation.

I'm working on polyfills for the Geomtry Interfaces at
github.com/trusktr/dommetry.

In my implementation, the `matrix.rotate(45).a` shows the same result as
the Firefox implementation in the console: `0.7071067811865475`. However,
the result of `matrix.rotateSelf(45).a` differs between my implementation
and Firefox. Mine shows `0.7071067811865475` and Firefox shows `
0.7071067690849304`.

Any idea why this is? Is this expected? Or might there be a bug somewhere?

*/#!/*JoePea