message: frame: Add buffer_mut function
  This allows getting a mutable `RgbaImage` from a mutable `Frame` to
  easily edit a frame sequence (e.g. a GIF image).
diff: @@ -83,6 +83,11 @@ impl Frame {
  &self.buffer
  }
  + /// Returns a mutable image buffer
  + pub fn buffer_mut(&mut self) -> &mut RgbaImage {
  + &mut self.buffer
  + }
  +
  /// Returns the image buffer
  pub fn into_buffer(self) -> RgbaImage {
  self.buffer
  