Make Mushroom Grove/Lit Mushroom Grove behave like forests, move-wise

Prefer the terrain type with the worst movement costs between the base
and the overlay.

This is a forward-port of the following two commits from the 1.12
branch:

 * 18282b542fe48614d74090535552ba4a9d430682 Fix bug #21971: visual glitch of terrains on 2p - Hornshark Island
 * 35a48f29f9134169b249af827730d373909e3562 Make Lit Mushroom Grove stats identical to Mushroom Grove