It'd be useful for us if PassAction::clear_color were const.

This PR marks everything as const that can be, by iterating

cargo clippy --fix -- -A clippy::all -A rust_2024_compatibility -A unused -W clippy::missing_const_for_fn

until a fixed point is reached.

That might be too aggressive, since making something const is an API commitment. For instance, making Icon::miniquad_logo const might make addressing #570 a breaking change. A subset of these changes are probably sane, though, and I decided it's more useful as a PR than an issue.