Removed OpenGLDevice.CurrentFramebuffer

Addressing the concern here: https://github.com/flibitijibibo/MonoGame/pull/227#issuecomment-44687310

Removing the property didn't seem too bad. I don't see any of the redundant state errors (so the original fix still works) and I was able to call GetBackBufferData successfully and get the expected data without any noticeable side effects (i.e. no flashing, etc). I don't have any videos to test the video player fix, but that change seems reasonable to me.