Change frames per second (fps) from usize to f32

[paulingalls]

Some videos have frame rates like 29.97 and so fps needs to be represented as a float.

[jamjamjon]

@paulingalls Thanks for the contribution — looks great overall!
Before merging, could you please run

cargo fmt --all
cargo clippy --all-features --all-targets -- -D warnings

so the CI checks can pass?

[paulingalls]

Looks like all the checks now pass. Thanks for having a look!