Code review comment for lp:~cemil-azizoglu/mir/egl-native-window-for-PCs

Also, I think that MirBufferStream shouldn't go away. It fits a need that the clients have (as well as being used extensively in the wild).

Made some design drawings:
A) How things are in current design:
https://docs.google.com/drawings/d/1U03QrxQwPFYVfryC_-xOb64D3GRYIkJCAyw8LEWbbIk

B) how things would be with mir_presentation_chain_get_egl_native_window_type(...):
https://docs.google.com/drawings/d/1cX54j6Wv7TRuWLCXRuUyni9RgyuljWGEAlalDirwAys

C) How things would be with mir_create_egl_native_window_type(...):
https://docs.google.com/drawings/d/1yze9GbDb4pbGyQM7pXPDWXqrToW9Ko3gaAtWt9DeV_Q

A) and C) keep the MirPresentationChain just a thin wrapper around the RPC (which is useful for building future technologies). IMO, A) is a bit better because we only have one way of supporting EGL.