Rasterization is slow

[jpreiss]

It takes several seconds to rasterize a ~100-slide deck. There is probably some low-hanging performance optimization fruit still:

• Rasterization happens in a background thread, but only one thread
• Rasterization starts over from files every time

It is tempting to display the first slide before they are all done rasterizing, but this would introduce the possibility of navigating to a slide that is not ready to show yet, which is the whole thing we're trying to avoid (vs. Preview).

[jpreiss]

It looks like pdf2image has a lot of constant overhead, so shelling out one page at a time is inefficient. Boosted performance a little with chunking. Also was busy-waiting on the queue too much.

Probably this is about the best we can do with pdf2image. The next step would be using a pdf parser library so we can only pay the read/parse penalty once, and rasterize the same data structure at different sizes. Not sure if Poppler does this or not, but probably.