Switch from accepting an &[u8] to exposing an &mut [u8]

[parasyte]

This is from a hint provided by Ralith:

you should arrange for your library to expose an &mut [u8] rather than taking a &[u8] as input; in the future this will allow you to save a copy

Two public API changes:

• Pixels::render(&mut self, &[u8]) to Pixels::render(&mut self)
• Add Pixels::get_frame(&mut self) -> &mut [u8]

Using the new API will look like this:

let mut pixels = Pixels::new(320, 240, surface_texture)?;

// ... in `EventsCleared` handler ...
invaders.update(&dt, &controls);
window.request_redraw();

// ... in `RedrawRequested` handler ...
invaders.draw(pixels.get_frame());
pixels.render();