add __multiply to metatable for mat4 (should just alias to multiply().

[FatalistError]

issue is the name. Putting this down for if I ever make a PR

[appgurueu]

As a heads-up, I'll probably write a proper unit-tested matrix module supporting variable dimensions (and probably using a flat table) for the algorithms repository anyways. Besides __multiply, __add, __sub and __unm should of course also be set (these are simple though). Applying a matrix to a vector should also be added.