event: lazify screen update

Set root screen change flag in event handler, and update in
draw_callback.

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

src/compton.h

@@ -25,6 +25,10 @@
 #include "win.h"
 #include "x.h"
 
+enum root_flags {
+	ROOT_FLAGS_SCREEN_CHANGE = 1
+};
+
 // == Functions ==
 // TODO move static inline functions that are only used in compton.c, into
 //      compton.c
@@ -62,6 +66,8 @@ void queue_redraw(session_t *ps);
 
 void discard_ignore(session_t *ps, unsigned long sequence);
 
+void set_root_flags(session_t *ps, uint64_t flags);
+
 /**
  * Set a <code>switch_t</code> array of all unset wintypes to true.
  */
@@ -151,5 +157,3 @@ static inline void dump_drawable(session_t *ps, xcb_drawable_t drawable) {
 	          drawable, r->x, r->y, r->width, r->height, r->border_width, r->depth);
 	free(r);
 }
-
-// vim: set et sw=2 :