Initialize jitter buffer delay with half of its size

Also add a function to reset jitter buffer

src/libjitter/jitter.h

@@ -6,6 +6,7 @@ typedef struct jitter {
 } jitter_t;
 
 int jitter_create(jitter_t *jitter, int length);
+void jitter_reset(jitter_t *jitter);
 void jitter_destroy(jitter_t *jitter);
 void jitter_save(jitter_t *jb, sample_t *samples, int length);
 void jitter_load(jitter_t *jb, sample_t *samples, int length);