Z ordering fixed. Grass tufts added.

Much nicer z ordering for objects.

Added some grass tufts. Only one sprite, a few variants would be nice.

Items at the bottom of the screen render above objects at the top of the
screen. Except wasps which render on top until killed, then use y
position to determine paint order.

Swatter and food are trickier - they take their z order value from the
platter and player respectively.