message: add explicit error for to_pixels on MSAA images.
  wgpu should already throw an error when this happens, but this should be more user friendly
diff: @@ -197,6 +197,12 @@ impl Image {
  ///
  /// **This is a very expensive operation - call sparingly.**
  pub fn to_pixels(&self, gfx: &GraphicsContext) -> GameResult<Vec<u8>> {
  + if self.samples > 1 {
  + return Err(GameError::RenderError(String::from(
  + "cannot read the pixels of a multisampled image; resolve this image with a canvas",
  + )));
  + }
  +
  let block_size = self.format.describe().block_size as u64;
  let buffer = gfx.wgpu.device.create_buffer(&wgpu::BufferDescriptor {
  @@ -345,9 +351,7 @@ impl ScreenImage {
  assert!(height > 0.);
  assert!(samples > 0);
  - let format = format
  - .into()
  - .unwrap_or(gfx.surface_format);
  + let format = format.into().unwrap_or(gfx.surface_format);
  ScreenImage {
  image: Self::create(gfx, format, (width, height), samples),
  