The matrix "apply" function

For converting a point trough the matrix. Returns a new point.
Returning a PIXI.Point
The function name shouldn't be "toGlobal" of course.

- Added comments.
Added a 2nd parameter, newPos

Added a 2nd parameter, newPos, so a new point isn't necessarily created.
Merge pull request #856 from lucbloom/matrix-apply-applyInverse

The matrix "apply" function