canvas: add ask_update() method to ask for an update on next frame.

(basically, it just call flag_update() method.)
+ use it in Video()

kivy/graphics/instructions.pyx

@@ -368,6 +368,13 @@ cdef class Canvas(CanvasBase):
         c.rremove(self)
         self.flag_update()
 
+    def ask_update(self):
+        '''Ask the canvas to update itself the next frame.
+        Can be useful when a texture content is changing, but anything else in
+        the canvas.
+        '''
+        self.flag_update()
+
     property before:
         '''Property for getting the before group.
         '''

kivy/uix/video.py

@@ -117,6 +117,7 @@ class Video(Image):
         self.duration = self._video.duration
         self.position = self._video.position
         self.texture = self._video.texture
+        self.canvas.ask_update()
 
     def _on_eos(self, *largs):
         self.eos = True