Migrated OpenGLESSample to unified API

- GL.Oes.GenFramebuffers(int, ref uint) to (int, out uint)
- GL.Oes.GetRenderbufferParameter(All, All, ref int) to (All, All, out
int)
- In NSTimer.CreateRepeatingScheduledTimer(TimeSpan, Action<NSTimer>),
added NSTimer as param to anonymous delegate Action<NSTimer>