Improve libf3d API in regards to time management

[mwestphal]

Describe the bug Improve libf3d API in regards to time management

There is currently no way in libf3d to:

• extract the timerange
• select a timestep and ONLY call LoadAtTime, not a complete loadScene

[Nokse22]

For Exhibit to be able to show animations I need to know the time range and if any animation is preset so #1628 would be needed. Then using scene.animation.time I can play animations controlling them from the app.

[snoyer]

select a timestep and ONLY call LoadAtTime, not a complete loadScene

For this one, as discussed a while back on Discord, there are 2 ways to go about it:

1. expose LoadAtTime() in the API somehow
2. have scene.animation.time not be a "load time" option, and internally call LoadAtTime() if the value of scene.animation.time has changed between calls to render()

The second one would be more convenient for users and doesn't require expanding the API, but there's a concern about silently triggering IO as some readers may require reloading the whole file.