Remove latest session option when generating images (to reduce file size)

[huntfx]

Generating an image for the last session was initially added because the tracks actually contained all the necessary information and it was a surprisingly easy tweak. At a later date, support for clicks was added, but at the cost of creating a load of empty arrays. As to an idea of the overhead, if the mtk file had tracks for 720p, 1080p and 1440p, then the session arrays contain over 20m integers. As well as increasing file size, it's all got to sit in memory.

Up until now I've not paid much attention, but with possible track types I'm experimenting with, I don't want to be adding separate session arrays for each one.

It would be useful to know what session data people would be interested in. I'll probably take the functionality out for the time being, but leave the issue here if anyone has any objections in the future.