Mobs are too stupid to walk around obstacles

[Wuzzy2]

Seems like animals are a bit stupid. If animals are attraced by something you hold in your hand, the mobs fail to find a way around obstacles. They try to walk the direct way. If a wall is in the way, they just walk into the wall and never reach the destination.

How to test:

• Build a wall made of glass blocks
• Spawn chicken behind the glass wall
• Stand in front of glass wall
• Select seed on your hand and look at chicken
• Wait

[kwadroke]

Similar to issue #389. Mobs don't know they can't jump any higher, but, try anyway. Sometimes they slide over and are successful when the ground allows (except sometimes rabbits if on snow), but, usually are just stuck until they de-spawn.

Running commit 34292ece10fd1d3f95bcd3cdba83df72e3a77255
See screenshots below.