Reworked chunk update limiting and fog.

1. Max number of chunk updates per frame is determined dynamically/adaptively when vsync/framerate-limit is on. Number of updates is determined by measuring the approximate amount of time each update takes and the amount of time vsync wastes idling. Extra updates are done when there is enough idle time.
2. Added 2 new fog settings (SHORTER and FARTHER) and reworked the other settings. Previously fog distances were {8,32,128,512}. Now they are {8,16,32,64,128,256,512,1024}. Fog now starts further away from the camera, to prevent a washed-out look. "Render distance" label in settings changed to "View distance".

Signed-off-by: fragmer <me@matvei.org>
Merge pull request #286 from fragmer/master

Reworked chunk update limiting and fog.