Fix regression of commit 0a7bf77b8c0765f8a53dc72a8edab8e0496938ff - GLReadBufferUtil's GLPixelBuffer* usage. Ref GLJPanel SingleAWTGLPixelBufferProvider's screenshots.

Ref GLJPanel SingleAWTGLPixelBufferProvider's screenshots:
  <http://jogamp.org/files/screenshots/gljpanels-singlebuffer/>