Correct mat4 and vec4 multiplication

Interesting bug; this fix aligns this function with the float32 version.
Also, this was me trying to figure out what was wrong with *my* code.

![](http://us.cdn3.123rf.com/168nwm/auremar/auremar1211/auremar121103266/16472149-frustrated-man-pulling-his-hair-out.jpg)