Fix 3d picking for rays that have a zero component

When using MOAIPartition:propListForRay() with a ray along the
Z axis (i.e. (0, 0, -1)), there were false positives in the result
buffer.

Fixed by checking against both min and max when the direction
component is (almost) zero.

I also updated the 3d picking sample to check if it still works
correctly.
Merge pull request #683 from theGoodEvil/fix_3d_ray_picking

Fix 3D Picking for Rays That Have a Zero Component