Fixed frustum culling not working. (thanks 123DMWM)

Shamelessly quoted from https://github.com/andrewphorn/ClassiCube-Client/commit/25410ec843d302eb8cb9647b58fbbb0935964f07#commitcomment-5639961: 
> The last 32 blocks along the X axis are invisible on a 512x512 map.

This should fix the issue. (and increase performance)