Proper animal pens

[Kosti]

We've all experienced the pain of a fenced off animal farm being wiped out by a newly spawned wolf/bear inside, or the beasts just sliding through the fence. Fix it please. 

For starters make sure that the fence works properly and that animals can't glitch through, then make it so that we don't need to put two blocks of fence atop each other, not because it's unrealistic to have this high a fence, it's quite literally ugly, regardless of the fence type. 
Now here's a thing, make it so that the area of the enclosure calculates the blocks beneath it in as well up to 1 block below, and sets them to be unable to spawn any mobs (excluding rifts).
Afaik these will be enough to fix the problems. 

Thanks for reading! 

[Facethief]

The fences cannot be pathed over/through by anything but a player, AFAIK, so single-high fences should be fine.

Not as sure on the deal with things clipping through fences, though.