Fix gs_matrix_* issues

Multiplication of the matricies was being done in the wrong direction.
This caused source transformations to come out looking incorrect, for
example the linux-xshm source's cursor would not be drawn correctly or
in the right position if the source was moved/scaled/rotated.  The
problem just turned out to be that the gs_matrix_* functions were
multiplying in the wrong direction.  Reverse the direction of
multiplication, and the problem is solved.