Overhauled window drawing

- Draw onto pixmap as before, but use the same size for the pixmap as
  for the window, allocate new pixmap after configure requests
- Use XCopyArea() instead of XSetWindowBackgroundPixmap(), which now
  requires handling of Expose events

window.h

@@ -66,6 +66,7 @@ void win_open(win_t*);
 void win_close(win_t*);
 
 bool win_configure(win_t*, XConfigureEvent*);
+void win_expose(win_t*, XExposeEvent*);
 bool win_moveresize(win_t*, int, int, unsigned int, unsigned int);
 
 void win_toggle_fullscreen(win_t*);