TD rendering freeze fix

This is a fix for the Lava Mountain freeze issue from TD (KartKrew/TD#13 (closed)) which can also occur in vanilla SRB2 as it turns out. Basically the issue occurs whenever an FOF with a single-patch texture with holes goes off the bottom of the screen far enough.

Looks pretty reasonable from code reasoning alone. There's no legitimate reason to enter the loop whilst things are like that, me thinks.

No problems here!

Status changed to merged

mentioned in commit 66a45c65