[dvdplayer/rbp] Add pi specific option to maintain vsync with pll adjustment

New A/V sync option in settings/video/playback to do "Adjust PLL".
This uses video clock (so perfect video syncing) but avoids having to resample
or drop/dupe audio packets which is normally required.

Needed updated firmware
[dvdplayeraudio] Add advancedsetting for configuring max pll adjustment
[dvdplayer] exerimental: don't raise priority of audio thread
[mmalcodec] Introduce a preroll period to buffer up frames on startup
[audio] Add settings option to boost centre channel when downmixing

This allows a dB volume increase to be added to centre channel.
This can help improve dialgue in the presence of background music/effects.
It can go up to 30dB for testing purposes, but value of 6 is probably more reasonable.
It is recommended to ensure "Normalise levels on downmix" is enabled when boosting by large values to avoid clipping.

Should work with Pi Sink (dvdplayer/paplayer) and omxplayer
[omxcore] Skip out of flush on error
[SinkPi] Handle multichannel layout more like OMXAudio
[rbp] Default extract thumbnails to false

It can take 80 seconds for a single file on a Pi. It can cause crashes with out-of-memory errors.
It genereates a lot of support issues. Best to default to disabled and let users enable it if they must
[cec] Add settings for configuring button repeats
[cec] Don't discard buttons when repeat mode is enabled
[cec] Temp - more logging
[resamplepi] Try to report the same numbers as ffmpeg
[dvdplayer] Add lock for player creation
[mmalrender] Skip some log message in bypass mode
[mmalcodec] Avoid deinterlace for dvd menus for now
[dvdplayervideo] Prod decoder when in stills mode

An asynchronous hardware decoder doesn't only produce output pictures when new packets arrive.
In dvd stills mode give it a chance to return pictures that weren't ready when frame was decoded.
[mmalcodec] Skip some setup that is not required when closing
[languageinvoker] Reduce priority of python threads
[mmalcodec] Try reducing number of video frames
[mmalcodec] Ensure we have requested number of buffers in non-accelerated case