[dvdplayer] Allow dvd menus to work with hardware decoders

Video from DVD menus (stills) are currently always software decoded (and don't use ffmpeg).
The understanding has been this is due to some custom settings not compatible with ffmpeg and hardware decoders. I don't believe this to be the case.

The real issue is that we only poke the decoder when a new packet arrives. Hardware codecs generally don't return a frame immediately, but on a later call. Will dvd stills there may not be a later call, which means the decoded picture never has a chance to be returned.

This PR also prods the decoder with in stills mode from the normal timeouts from message queue to give it a chance to return pictures.

The first two commits just refactor and should have no functional change.
The third (minor) change makes it work.