Fix PulseAudio audio format to OBS audio format mapping

The format PA_SAMPLE_S24_32LE is a 24 bit audio format in 32 bit integers
and not a 32 bit audio format and so it should no be mapped to
AUDIO_FORMAT_32BIT.
Force PulseAudio sample format if the source format is not supported by OBS

If the sample format used by PulseAudio can not be converted into an
OBS audio format it will be handled as AUDIO_FORMAT_UNKNOWN which will
not result in a proper audio recording. So instead we request a format
that OBS supports from PulseAudio and let it do the format conversion.
Merge pull request #276 from reboot/fix_pulse_audio_format_conversion

Fix pulse audio format conversion