Wrap root window background prop check

It might be reused by the backends.

Depends on what the backend API ends up look like.

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

src/x.h

@@ -164,6 +164,6 @@ xcb_pixmap_t x_get_root_back_pixmap(session_t *ps);
 
 /// Return true if the atom refers to a property name that is used for the
 /// root window background pixmap
-bool x_atom_is_background_prop(session_t *ps, xcb_atom_t atom);
+bool x_is_root_back_pixmap_atom(session_t *ps, xcb_atom_t atom);
 
 bool x_fence_sync(xcb_connection_t *, xcb_sync_fence_t);