optimization: Render on pixmaps and copy the result on Stack_Wins

This should speed up the rendering of Stack_Wins with many window
decorations and it should considerably reduce flicker.

include/i3.h

@@ -30,6 +30,7 @@ extern TAILQ_HEAD(assignments_head, Assignment) assignments;
 extern SLIST_HEAD(stack_wins_head, Stack_Window) stack_wins;
 extern xcb_event_handlers_t evenths;
 extern int num_screens;
+extern uint8_t root_depth;
 extern xcb_atom_t atoms[NUM_ATOMS];
 
 #endif