Make thumbnail generation threaded

[gregzaal]

Much of the time seems to be spent loading and reading the hdr file, which uses only one thread. Multiple thumbnails could be generated simultaneously.

[gregzaal]

Main concern is memory limitation - huge HDRIs can fill up all the memory during thumbnail generation (since it has to be stored in memory several times) - maybe there's a way to check how much memory is available and only add to the thread pool when there is enough available.

[gregzaal]

This can wait until the next release - for now just getting thumbnails from a server (#35) will be enough.

[gregzaal]

It's already threaded...