WM: window draw callbacks and split preview snap

This moves window overlay from hard coded flags into drawing callbacks.

It also supports snapping (holding Ctrl).

source/blender/windowmanager/WM_api.h

@@ -506,6 +506,11 @@ void		WM_progress_set(struct wmWindow *win, float progress);
 void		WM_progress_clear(struct wmWindow *win);
 
 			/* Draw (for screenshot) */
+void        *WM_draw_cb_activate(
+                    struct wmWindow *win,
+                    void(*draw)(const struct wmWindow *, void *),
+                    void *customdata);
+void        WM_draw_cb_exit(struct wmWindow *win, void *handle);
 void		WM_redraw_windows(struct bContext *C);
 
 void        WM_main_playanim(int argc, const char **argv);