16-bit PNG support, texture splatting, etc

I've added an exporter for writing the heightfield as 16-bit PNG images using libpng. It wasn't too hard after all.

Also, I think I've finished the implementation of texture splatting, and I've even written the tutorial.

If you've been waiting for these features, you won't have to wait too much longer. There are only a few bug-fixes and behind-the-scenes things to do before release 2.3b is ready.