message: Update Image documentation a little
  Just realized that Image is cheap to clone which makes life easier for things such as shoving an image into a sprite batch.
diff: @@ -1252,6 +1252,10 @@ where
  /// In-GPU-memory image data available to be drawn on the screen,
  /// using the OpenGL backend.
  +///
  +/// Note that this is really just an `Arc`'ed texture handle and some metadata,
  +/// so `clone()`'ing it is quite cheap since it doesn't need to actually
  +/// copy the image data.
  pub type Image = ImageGeneric<gfx_device_gl::Resources>;
  impl Image {
  