Interpolation over non FixedUpdate frames

[ThierryBerger]

• In the context of bevy task to run physics dimforge/bevy_rapier#596 ; I'll be looking into interpolate a position over multiple frames.

The intended way is to modify fixed update to match the simulation time as best as we can.

What I dislike about this is that we're still tied to the framerate, and I don't know when my fixed update will run. they are not part of a schedule.

Currently my implementation still runs in fixed update, but the simulation step is read "when done", which results in non-predictible "render frame" to "fixed frame" relation. We could mitigate that by explicitly waiting, but that's another subject.

A few ideas I plan to explore:

• be able to customize when the systems currently hardcoded in FixedLast and FixedFirst happen ?
• make a specific schedule which is run only when the simulation takes place? (a set is not enough because it would be run regardless ?
• User sends an event/trigger/system to signify an update frame has been computed.
• add a run condition on sets ; I think that doesn´t work with the FixedTime handling.

I think the best option is a custom scheduler with custom Time, like the Time<Physics> of Avian, but this crate should be able to play well with it if Time<Fixed> and Time<Physics> deviate, which is not the case for Avian with interpolation support (Time<Physics> is an alias to underlying time (Fixed for interpolation support)