qtaudio: Add Qt 6 support

[radioactiveman]

Draft status, still missing:

• periodSize() is not available in Qt 6. What can we use instead?
• How do we need to convert the audio data for signed/unsigned formats and endianess?

Qmmp uses:

#if Q_BYTE_ORDER == Q_BIG_ENDIAN
    if(format == Qmmp::PCM_S16LE)
        format = Qmmp::PCM_S16BE;
    else if(format == Qmmp::PCM_S32LE)
        format = Qmmp::PCM_S32BE;
#endif

From https://doc.qt.io/qt-6/qaudioformat.html#SampleFormat-enum:

Qt will always expect and use samples in the endianness of the host platform. When processing audio data from external sources yourself, ensure you convert them to the correct endianness before writing them to a QAudioSink or QAudioBuffer.

@jlindgren90, @kaniini: Could you please support here?

[jlindgren90]

How do we need to convert the audio data for signed/unsigned formats and endianess?

I don't think the plugin needs to do any conversions. Just make open_audio return false for the non-native (byte-swapped) formats. Audacious core already tries to use the native formats IIRC.

Not sure about the period size thing. Generally I don't think Audacious core cares about the period size as long as period_wait works.

[jlindgren90]

Looks good. Thanks!