A mount child is an EntityKind::Unit but rides its host, so its Transform is
host-local (0,0,0). ground_clamp_creatures cast a world ray from that origin and
wrote the world floor height into the child's local Y, dropping the mount into
the terrain; in Alterac Valley the map origin sits on live ground, so mounted
players showed half under the map. ground_clamp_creatures and
mark_swimming_creatures now exclude MountBody, as every other mount-aware system
already does.
Fixes#4
chairmanpow
2796a5d78d