Most of the x_* functions don't need session_t

Replace session_t parameter with xcb_connection_t if that's the only
thing needed.

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

src/compton.h

@@ -154,7 +154,7 @@ dump_drawable(session_t *ps, xcb_drawable_t drawable) {
 static inline void
 win_validate_pixmap(session_t *ps, win *w) {
   // Destroy pixmap and picture, if invalid
-  if (!x_validate_pixmap(ps, w->paint.pixmap))
+  if (!x_validate_pixmap(ps->c, w->paint.pixmap))
     free_paint(ps, &w->paint);
 }