twn_audio: rework interface

src/twn_audio_c.h

@@ -36,11 +36,13 @@ union AudioContext {
 
 
 typedef struct AudioChannel {
-    PlayAudioArgs args;
     AudioFileType file_type;
     union AudioContext context; /* interpreted by `file_type` value */
     const char *path;
     const char *name;
+    bool repeat;
+    float volume;
+    float panning;
 } AudioChannel;