Think about frame number

Right now, `Time::frame` is an `f64`, but is called a frame number. It used to be 32 bit, but was changed.

Most of the time, in other systems, a frame number is an integer.

Some times, this value is a percentage of the duration rather than a frame number. Other things treat it as a number of fractional seconds since the start of the animation.

This is an area that deserves further thought.

Start by locating the definition and uses of `Time::frame` in the Rust project. Compare whether callers treat it as an integer frame number, a duration-relative percentage, or fractional seconds, and document the intended meaning before changing the API.

Written by the indexing model from the issue text.