The rendering engine is now detached from the SDL viewer

As I mentioned in a previous post, I wanted to detach the sparse voxel octree rendering code from the SDL based viewer. With commit f36fc4ea that is now realized. The rendering engine now only requires the GLM library, which is used to setup the initial call to traverse. The engine has libpng as an optional dependency. If you have it, the engine will provide a method with which you can easily save the resulting image as png, otherwise that method is a no-op. The benchmark and viewer still require SDL though and are not build if not available.