Grid node

This adds a node for generating grids - I plan to use it to implement a grid in the viewer, but it seems worthwhile having it as a scene source too. It also makes a very good example for learning to write scene sources, as it does a little bit of everything (hierarchy, attributes, objects, transforms, bounds).

I've tried to compress the constant.frag -> Constant.frag rename into one commit on both OSX and Linux and failed miserably. Sorry about that - I don't really understand the problem properly, but it's because I first tried to do it on OS X, where the filesystem is case insensitive.