pcm_export: add option "pack"

Converts padded 24 bit samples to packed 24 bit samples.  Will replace
the packed S24 sample format, which is not used internally.

src/output/alsa_output_plugin.c

@@ -525,7 +525,7 @@ configure_hw:
 	ad->period_position = 0;
 
 	pcm_export_open(&ad->export, audio_format->format,
-			reverse_endian);
+			false, reverse_endian);
 
 	return true;
 

src/output/oss_output_plugin.c

@@ -536,6 +536,7 @@ oss_probe_sample_format(int fd, enum sample_format sample_format,
 
 #ifdef AFMT_S24_PACKED
 	pcm_export_open(export, sample_format,
+			false,
 			oss_format == AFMT_S24_PACKED &&
 			G_BYTE_ORDER != G_LITTLE_ENDIAN);
 #endif