Signature for callbacks reporting that an image is available.
Used by ImageStream.
synchronousCall argument is true if the listener is being invoked
during the call to addListener. This can be useful if, for example,
ImageStream.addListener is invoked during a frame, so that a new rendering
frame is requested if the call was asynchronous (after the current frame)
and no rendering frame is requested if the call was synchronous (within the
same stack frame as the call to ImageStream.addListener).
typedef ImageListener = void Function(ImageInfo image, bool synchronousCall);