Added OpenCL support for GPGPU chunk generation. On my machine (GTX 960) this makes the best case massively better, but the worst case massively worse, so YMMV. OpenCL configurations will run at 4x resolution.

If you don't see an (OpenCL) configuration the config select screen your PC doesn't expose OpenCL, sorry!

There's also some other fixes:
- Removed dependency on VS CRT dll
- Fix lighting on recent NVidia drivers
- Rejig HDR values for more detail in the dark end
- Improved rough materials
- "Super low detail" to speed up distant lods
- Added pumpkin tool
- Fix issues with freehand
- Fix quantisation of mod placement