invalidate-bitmap-cache

When on-paint is overridden, call this method when the state of on-paint's drawing changes.

• (send a-wx:media-buffer invalidate-bitmap-cache x y width height) -> void
      x = 0.0 : inexact real
      y = 0.0 : inexact real
      width = -1.0 : inexact real
      height = -1.0 : inexact real

  The x, y, width, and height arguments specify the area that needs repainting in buffer coordinates. If width/height is -1, then the total heigh/width of the buffer is used.