Use consuming builder pattern for SpritesheetBuilder

[flother]

Change SpritesheetBuilder methods from &mut self -> &mut Self to mut self -> Self. Allows for cleaner method chaining. Before:

let mut spritesheet_builder = Spritesheet::build();
spritesheet_builder.sprites(sprites);
spritesheet_builder.spacing(spacing);

After:

let mut spritesheet_builder = Spritesheet::build()
    .sprites(sprites)
    .spacing(spacing);

I've added #[must_use] to SpritesheetBuilder so that there's a compile-time warning if a builder is created but not used.