Fixed invalid OpenGL operation error caused by calls to glReadPixels(GL_BACK)....

Fixed invalid OpenGL operation error caused by calls to glReadPixels(GL_BACK). With Qt5 QOpenGLWidget, OpenGL commands are draw into offscreen frame buffers so that is probably why the GL_FRONT and GL_BACK buffers do not exist..  Thus, do not call glReadPixels().  Identification operations continue to successfully complete.