Added explicit glFlush when eglSwapBuffers is invoked on devices running with iOS version strictly prior to 6.0

The spec states: "eglSwapBuffers  perform an implicit flush operation on the context (glFlush for an OpenGL ES context)."

[EAGLContext presentRenderBuffer: ... ] does not automatically call glFlush on pre iOS 6.0 while it does on 6.0 and above.