Scaffolding blocks spawning

The bug

Mobs cannot spawn with the top of a scaffolding block touching their hitbox. For example, a zombie cannot spawn if there would be a scaffolding block covering its eyes. However, the scaffolding would not block it's movement, so it should be able to spawn inside.

Since creepers and spiders are not tall enough, they spawn anyways.

This would also provide some interesting new possibilities for mob farms.