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.
Add a handler for the OpenAPI `taskOutputProduced` operation, and an
image thumbnailing goroutine.
The queue of images to process + the function to handle queued images
is managed by `last_rendered.LastRenderedProcessor`. This queue currently
simply allows 3 requests; this should be improved such that it keeps
track of the job IDs as well, as with the current approach a spammy job
can starve the updates from a more calm job.
