ios/GLView: render buffer only after something has been drawed

Issue: An OpenGL ES-based application displays "flashing" or "stale"
frames after a call to presentRenderBuffer.

This symptom can occur when an OpenGL ES application calls the EAGL
presentRenderbuffer method without first drawing anything. What is
seen on screen may contain uninitialized pixels or previously rendered
frames. To correct this issue, you should always draw something to your
framebuffer before calling presentRenderbuffer. Also note that unless
you set the RetainedBackbuffer property on your CAEAGLLayer to enable
retained backbuffer mode, the contents of your renderbuffer are not
guaranteed to remain valid after a call to presentRenderbuffer.