Don't pass cairo surfaces around on the lua stack

Don't pass cairo surfaces around on the lua stack

Now that the C code uses lightuserdata for passing around cairo surfaces, they
are no longer automatically garbage collected. To avoid memleaks, this commit
compares the C code to use cairo_surface_t pointers instead of the lua stack.

This also fixes a memleak were a client's icon was leaked.

Signed-off-by: Uli Schlachter <psychon@znc.in>