Fix "missing face" bug

[fgerlits]

If a 2-dimensional face lies in the hyperplane but neither of the two adjacent 3-dimensional faces do, we don't make this face part of the intersection. This can result in an intersection with a hole in the middle. There is already a FIXME about this in ThreeDimensionalFace.java.

To reproduce: from the initial position, turn the xy slider to exactly (+ or -) 45°. This is where the color of the face changes. If you find the point at exactly 45°, you'll get a tunnel with a hole in the middle.

Proposed solution: in cases like this, draw the face with a mixture of the colors of the two 3-dimensional faces. This would also help in the case when a complete 3-dimensional face is in the hyperplane: the cube would no longer be all the same color, so hard to see.