mvminer can start asynchronous jobs

Before today, mvminer was a single-threaded program. Today, mvminer has a job queue and can load chunks in the background! Gone are the days of waiting several minutes for the (horribly slow) texture generator to run 1,048,576 times before the display comes up.

Mapping tiles into the virtual texture and loading vertices cause a slight pause while they block the GL thread, so I will build rate limiters into their classes. Right now the virtual tile map and shared vertex buffer can only be operated from the GL thread; they should buffer changes and submit them in reasonably-sized batches per frame.

Here are a few screenshots taken as chunks load around the origin:


mvminer has started loading


oh my this texture generator is slow


mvminer continues to load

As always in the first undesigned iteration, this implementation is a complete hack (the asynchronous jobs are all queued up on startup and aren't owned -- quitting before they finish causes a segfault as their working sets are reclaimed by the system).

Once the code is cleaner again I will need to start on a "loading manager" (not sure of the clearest name for it). This system will maintain a prioritized record of nearby chunks and, if they are loaded, their exposed tile counts. It will issue all of the load/save/map-tiles/unmap-tiles requests.