gl-renderer: Make clip_transformed() surf parameter constant

Calling clip_transformed() 4 times in a row with the same polygon8 in
commit a4d31fa8 introduced a bug
because the surf input is modified each time. This is fixed by working
on a local copy. The input parameter is marked constant to reflect the
change on the function prototype.

Fixes #764

Signed-off-by: Loïc Molinari <loic.molinari@gmail.com>