PlaneGeometry: Changed faces to use Face3 instead of Face4.

This enables raycasts to properly intersect with it in cases where the
positions are modified.  See
http://www.realitymeltdown.com/WebGL/Debug-Raycast.html for an example
illustrating the problem.

If this is not accepted, please tell how I can make it acceptable like should it use Face4 by default and have an option to use Face3?