imagebox: Redraw in :set_image()

If someone modifies a cairo surface and then sets the resulting object as the
image of an imagebox, the imagebox needs to redraw. Thus, since surfaces are
modifiable, we cannot assume that nothing changed when the same image is set
multiple times on an imagebox.

However, the dimensions of a surface cannot be changed and thus this does not
need to emit widget::layout_changed.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/wibox/widget/imagebox.lua

@@ -96,6 +96,8 @@ function imagebox:set_image(image)
     end
 
     if self._image == image then
+        -- The image could have been modified, so better redraw
+        self:emit_signal("widget::redraw_needed")
         return
     end