Voxel artifacts with infinitely thin shapes

[lilleyse]

The voxel code allows for creating infinitely thin shapes, which is a handy feature for inspecting individual slices of voxel data. There are some artifacts though:

• When making a shape infinitely thin via min/maxBounds it always renders the front side. Instead it should render just the side that was hit. This happens for most shape types. Note: this bug only happens for min/maxBounds, not min/maxClippingBounds. See message previously in convertUvToEllipsoid.glsl:

    #if defined(ELLIPSOID_HAS_SHAPE_BOUNDS_HEIGHT_FLAT)
        // TODO: This breaks down when minBounds == maxBounds. To fix it, this
        // function would have to know if ray is intersecting the front or back of the shape
        // and set the shape space position to 1 (front) or 0 (back) accordingly.
        float height = 1.0;

• Ellipsoids only render one side of the shape when the latitude range is zero. This happens for both min/maxBounds and min/maxClippingBounds.