message: flush the draw command buffer before serializing a canvas
  fixes
diff: @@ -154,6 +154,11 @@ where
  /// Dumps the flipped `Canvas`'s data to a `Vec` of `u8` RBGA8 values.
  pub fn to_rgba8(&self, ctx: &mut Context) -> GameResult<Vec<u8>> {
  + // first flush the command buffer to make sure the canvas is up to date before being serialized
  + // fixes https://github.com/ggez/ggez/issues/813
  + let gfx = &mut ctx.gfx_context;
  + gfx.encoder.flush(&mut *gfx.device);
  +
  let mut pixel_data = self.image.to_rgba8(ctx)?;
  flip_pixel_data(
  &mut pixel_data,
  