[+][SUG][PL] Show hatch fall distance in CPM

[WillLem]

This idea is courtesy of Armani, who has implemented it his custom NeoLemmix build.

In Clear Physics Mode, we should show a fall distance ruler beneath each entrance hatch. The ruler can always the exact fall distance (63px) just in case terrain needs to be removed from beneath the hatch when lemmings are still spawning.