Code review comment for lp:~raof/mir/cursor-suitability-is-per-buffer

Note that we now don't throw any errors if someone tries to submit a non-argb-8888 buffer to a cursor stream.

That said, we didn't correctly enforce that before - someone setting up a presentation chain could happily submit non-argb buffers to it.

This is something that should be fixed, but it might want to be fixed by supporting non-argb8888 buffers for cursors.