setMatrix function

I think I mentioned on reddit that we're exporting Flash animations and running them in Futile. At first we just used position/rotation/scale, but we found that not having skews made life harder for animators and ended up with more jittery animation. 

I added skews back, but that required this small change to Futile.

When setting a transform matrix directly, the _didSetMatrix flag is set to true
Setting any other property (scale,rotation,position) unsets this flag and reverts to the previous behavior
While _didSetMatrix is set to true, UpdateMatrix and similar calls will not re-write the matrix