plugins/automatons: Fix OOB coordinate access in Grass.

Fixes one misbehaving test.

Refs #423.
plugins/physics: Fix OOB access in Fluid.

Pervasive little bug.

Refs #423.
plugins/physics: Fix another OOB access in Fluid.

Fixes another test, and with this, all of the OOB accesses that we test for
are now fixed.

Fixes #423.
motd: Patches welcome!