cogl-matrix-stack: fix use after free in matrix stack.

If the matrix was reallocated we would use values from the stack
for the matrix parameters. This fixes that and also uses the
function instead of out of lining the same code.

https://bugzilla.gnome.org/show_bug.cgi?id=671985



Reviewed-by: Robert Bragg <robert@linux.intel.com>
Reviewed-by: Neil Roberts <neil@linux.intel.com>