Fix pushMatrix and matrix multiplication

__mul was taking a self argument, made rhs nil.
__mul also has to re-wrap itself so the user does not get access to the real VMatrix type (as it is sensitive)

Furthermore pushMatrix did not work properly. With my changes it works, though I have no clue why. The real mul and the wrapped mul both work fine.
Merge pull request #47 from Xandaros/patch-1

Fix pushMatrix and matrix multiplication