Move session_t::pictfmts into x.c

As a global variable, since they shouldn't change during the period
compton is running. Also limit the scope of the variable to x.c.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/compton.c

@@ -3161,7 +3161,6 @@ session_destroy(session_t *ps) {
   }
   free(ps->o.glx_fshader_win_str);
   free_xinerama_info(ps);
-  free(ps->pictfmts);
 
   deinit_render(ps);