Offset Middleground Y
Changes the middleground's current Y offset to a new value.
Notes
Can be animated using an Easing for a given Move Time.
The offset is applied on top of the middleground's base vertical position and is not reset if the middleground's Y speed changes.
Y offset is given in 1/10th of a block steps.
Only one Offset Middleground Y trigger can be active at a time, new activations stop previous ones.