bar: don't use cairo

This is trivial in the Wayland backend; just instantiate a pixman
pointing to the same mmapped memory as the wayland buffer.

In the XCB backend, we change the implementation slightly; instead of
rendering via a cairo XCB surface backend (to a server side pixmap),
which is then blitted to the window in commit(), we now render to a
client-side pixman pixmap, and blit it using xcb_put_image() in
commit().

bar/backend.h

@@ -11,8 +11,7 @@ struct backend {
                  void (*expose)(const struct bar *bar),
                  void (*on_mouse)(struct bar *bar, enum mouse_event event,
                                   int x, int y));
-    pixman_image_t *(*get_pixman_image)(const struct bar *bar);
-    void (*commit_pixman)(const struct bar *bar, pixman_image_t *pix);
+    void (*commit)(const struct bar *bar);
     void (*refresh)(const struct bar *bar);
     void (*set_cursor)(struct bar *bar, const char *cursor);
 };