pcm_channels: use pcm_buffer

Replace a "static" buffer with the PCM buffer library.

src/pcm_channels.h

@@ -22,13 +22,17 @@
 #include <stdint.h>
 #include <stddef.h>
 
+struct pcm_buffer;
+
 const int16_t *
-pcm_convert_channels_16(int8_t dest_channels,
+pcm_convert_channels_16(struct pcm_buffer *buffer,
+			int8_t dest_channels,
 			int8_t src_channels, const int16_t *src,
 			size_t src_size, size_t *dest_size_r);
 
 const int32_t *
-pcm_convert_channels_24(int8_t dest_channels,
+pcm_convert_channels_24(struct pcm_buffer *buffer,
+			int8_t dest_channels,
 			int8_t src_channels, const int32_t *src,
 			size_t src_size, size_t *dest_size_r);