code: public Matrix4 setToFrustum (
          double left, double right, double bottom, double top, double near, double far) {
          return setToFrustum(left, right, bottom, top, near, far, Vector3.UNIT_Z);
      }
docs: Sets this to a perspective projection matrix. The formula comes from the OpenGL
  documentation for the glFrustum function.
  
  @return a reference to this matrix, for chaining.