fixed: check context is active before calling glGetError in frame exit      : allow frame exit glError check to by disabled by env var OSG_NO_GL_CHECK_ON_FRAMEEXIT
fixed: check context is active before calling glGetError in frame exit
     : allow frame exit glError check to by disabled by env var OSG_NO_GL_CHECK_ON_FRAMEEXIT

jondo2010/OpenSG,jondo2010/OpenSG,jondo2010/OpenSG,jondo2010/OpenSG,jondo2010/OpenSG