Add API to PlaybackBuffer to compute amount of data that can overwritten

The distance is between a given offset in the buffer (probably a
read position at some point in time) and the write ptr. Any data after
the write ptr is "old" and not readable, and thus not worth overwriting
since we would not read it anyway.

libs/pbd/pbd/playback_buffer.h

@@ -122,6 +122,18 @@ public:
 		}
 	}
 
+	/* write thread */
+	guint overwritable_at (guint r) const {
+		guint w;
+
+		w = g_atomic_int_get (&write_idx);
+
+		if (w > r) {
+			return w - r;
+		}
+		return (w - r + size) & size_mask;
+	}
+
 	/* read-thead */
 	guint read (T *dest, guint cnt, bool commit = true, guint offset = 0);