Fix levels having incorrect `GlobalTransform` the first frame

[johanhelsing]

The transform propagation system runs in the PostUpdate stage, so spawn the levels before PostUpdate so they have time to propagate, but after ProcessApi, so they'll be ready the next frame (assuming the LdtkWorldBundle will be spawned during the Update) stage.

I ran into this bug because in my game I scale the levels, and collision detection is therefore way off during the first frame sending my player character flying.