Code review comment for lp:~kdub/mir/nested-buffer

quick sketch:
native_buffer_base() needs to provide gl support so the nested server can use the buffers for GL composition. The different platforms have different EGLImage import methods, still working out how that will work.

native_buffer_handle() needs to provide a mg::NativeBuffer that gives mgn::DisplayBuffer access to the MirBuffer* so it can use MirPresentationChain to forward the MirBuffer* along without composition.