settings: Add acceleration option for resampling
[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
[vobsub] Fix for incorrect colours with vobsubs and GLES

Fixes #15146

The code provides the overlay texture in BGRA format, which it informs GL about.
However GLES does not support BGRA, and just claims it is RGBA which results in a red/blue swap.

So this code path needs a red/blue swap.