Add automated benchmarks

[mattico]

It would be nice to have automated and repeatable benchmarks in hematite. I see two ways of accomplishing this, and it might be nice to have both:

1. Abstract the render loop into functions and then call these functions inside some standard #[bench] tests. This method is limited by the simplicity of the Bencher API, but it can still be useful.
2. Have a benchmark mode similar to many games that records statistics while the camera goes through a predefined path.