wibox.background: Add shape clip support

lib/wibox/widget/background.lua

@@ -72,6 +72,12 @@ function background:before_draw_children(_, cr)
     if self.foreground then
         cr:set_source(self.foreground)
     end
+
+    -- Clip the shape
+    if self._path and self._shape_clip then
+        cr:append_path(self._path)
+        cr:clip()
+    end
 end
 
 --- Layout this widget
@@ -155,6 +161,13 @@ function background:set_shape_border_color(fg)
     self:emit_signal("widget::redraw_needed")
 end
 
+--- When a `shape` is set, make sure nothing is drawn outside of it.
+-- @tparam boolean value If the shape clip is enable
+function background:set_shape_clip(value)
+    self._shape_clip = value
+    self:emit_signal("widget::redraw_needed")
+end
+
 --- Set the background image to use
 -- If `image` is a function, it will be called with `(context, cr, width, height)`
 -- as arguments. Any other arguments passed to this method will be appended.