one should not have to set allow_overlay to get promotion hints.  want_promotion_hint should be sufficient.  otherwise, one has to set surface_texture + allow_overlay + want_promotion_hint, which doesn't work well if one forgets to update something that copies the flags around (e.g., rtc_video_decoder).

not sure how easy this is to do.  i added the 'surface_texture' flag because it wasn't clear that it would work otherwise.  however, looking at it now, it's probably just a bit of fiddling to the overlay strategy, since it enumerates all the quads anyway.