Fix the image files not being freed, and remove the unused source
definition function 'update', which was additionally calling
image_source_load with the wrong number of parameters.
With the "instant-replay.lua" script, it would only play the last replay
buffer saved, meaning the user would have to save a replay with the
replay buffer hotkey, and then press the "Play Last Replay" hotkey to
play the last replay buffer.
Instead, combine the two and make the script save the replay
automatically first, and then play it back as soon as it's available.
This makes it actually qualify as an "instant replay" script, as its
name implies.
