public Vector3f getWorldCoordinates(Vector2f screenPos, float projectionZPos) {
        return getWorldCoordinates(screenPos, projectionZPos, null);
    }
Computes a position in World space given a screen position in screen space (0,0 to width, height)
and a z position in projection space ( 0 to 1 non-linear).
This former value is also known as the Z buffer value or non-linear depth buffer.
For more explanation of non-linear Z buffer see
http://www.sjbaker.org/steve/omniv/love_your_z_buffer.html

<p>To compute the projection space z from the view space z (distance from cam to object)
@see Camera#getViewToProjectionZ

@param screenPos 2d coordinate in screen space
@param projectionZPos non linear z value in projection space
@return the position in world space.
jme3-core/src/main/java/com/jme3/renderer/Camera.java