Add input and bounding shapes support (#2742)

Basic idea: if the window has a shape, set the parent container shape as
the union of the window shape and the shape of the frame borders.

Co-authored-by: Uli Schlachter <psychon@znc.in>

include/x.h

@@ -137,3 +137,8 @@ void x_set_warp_to(Rect *rect);
  *
  */
 void x_mask_event_mask(uint32_t mask);
+
+/**
+ * Enables or disables nonrectangular shape of the container frame.
+ */
+void x_set_shape(Con *con, xcb_shape_sk_t kind, bool enable);