patch from Tomaz adding a proper matrix inverter, it should be able to invert any...

patch from Tomaz adding a proper matrix inverter, it should be able to invert any matrix that is possible to invert, but is VERY slow, and consequently Matrix4x4_Invert_Simple has been kept for speed reasons
it is probably possible to optimize this function significantly, and it would also be possible to add non-uniform-scaling inversion to Matrix4x4_Invert_Simple to make it more flexible

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6759 d7cf8633-e32d-0410-b094-e92efae38249