After processing an image in the "last-rendered" processor, a SocketIO
object is sent to clients to indicate the last-rendered image needs to
be (re)loaded.
This also moves the previously existing "done callback" from a single
function to a per-image callback, so that it can be called with the
right information in there, and only when that particular image is
actually done processing.
The notification message sent via SocketIO also contains the necessary
info to render the image, so that the web client doesn't have to call
the `fetchJobLastRenderedInfo` operation.
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