Add 3D physics interpolation demo

[lawnjelly]

Demonstrate first person shooter and third person shooter cameras.

How to do FPS and TPS cameras with physics interpolation comes up regularly, and could do with a demo.

2025-06-24.15-06-44.mp4

Notes

• Just read all the requirements, I haven't the strength to edit any more today.
• Anyone else feel free to take this over.
• This is likely 4.5 onward (as the SceneTreeFTI changed after 4.4, and 4.4 is no longer recommended)

[Calinou]

Tested locally, it works as expected. This could also effectively act as a more minimal replacement of #849, although I think #849 still has its place as a more full-featured demo.

The demo only works correctly in 4.5.beta1, not 4.4.1.stable (most things aren't interpolated there, except bullets).

Some feedback:

• The folder should be renamed to just physics_interpolation, since it's already in the 3d folder.
• The 3D physics engine should be switched to Jolt Physics in the Project Settings to match existing demos.
• Bullets' collision layer should be changed (as well as the player's collision mask) so that the player doesn't collide with its bullets. This can cause it to suddenly jump if you shoot while moving forward.
• Friction on player movement should only be applied to the X and Z axes. Otherwise, the player slowly descends after a jump as if they're floating. This may require increasing gravity or reducing jump height to compensate (possibly both).
• There should be a shortcut action to toggle physics interpolation, so that you can see its effect in action. It would remain enabled by default nonetheless.

There are a few other changes I'd like to make. If you want, I can open a new PR based on this one.

[lawnjelly]

There are a few other changes I'd like to make. If you want, I can open a new PR based on this one.

Yeah that would be great! 👍

The particle texture BTW is from one of the other demos as I just needed a texture. That can be anything sensible.