Enhance bevymark

- Use `argh` for command line interface options
- Use seeded StdRng for reproducible random number generation
- Add a mode for testing 2D meshes that includes an option to uniquely vary the
  data of each material by setting a random flat colour on the ColorMaterial.
- Add a way of specifying the number of different textures to use for sprites
  or meshes. These are generated at the same resolution as the Bevy bird icon,
  but are just random flat colours for testing.
- Add a benchmark mode that spawns all entities during setup, and animates the
  entities using a fixed delta time for reproducible animation. The
  initially-spawned entities are still spawned in waves and animated as they
  would have been had they spawned at intervals.