Improve visual interpolation for examples

[cBournhonesque]

The current flow of visual interpolation for examples is:

• visually interpolate Position/Rotation

• to make sure that changes in Position are reflected in Transform, we:

  • put the avian SyncPlugin in PostUpdate (so that even if FixedUpdate doesn't run, we still have visual interpolation run, and then the Position/Rotation gets synced to Transform)
  • add the ordering in PostUpdate: VisualInterpolation -> Sync -> TransformPropagate
  • make sure that UpdateVisualInterpolation runs after PhysicsSet::Simulation in FixedPostUpdate

  All of this is pretty convoluted and very error-prone, instead what we want todo:

  • visually interpolate Transform (for this we need to make sure that an interpolation fn has been registered for Transform)
  • we don't need to touch the SyncPlugin; Pos/Rot are synced to transform in PostUpdate

This is much simpler!
As an extra optimization we can move SyncPlugin to RunFixedMainLoop so that it doesn't run on rollbacks.

Also add visual_interpolation to avian_physics example.