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

an interesting way that this branch manifested itself was that it showed that mir_surface_get_current_buffer() always returned the MirBufferPackage, which is the GBM platform's native type. This branch adds a new function to the client api, mir_surface_get_platform_type, which allows the user of mir_surface_get_current_buffer() to interpret the buffer they get from this function correctly.

This is really only used by xmir at the moment, so I'm working on updating xmir to support the api change