input_plugin: add method update()

Update the struct attributes, important for facades like the "rewind"
plugin.  To replace buffer().

src/input_stream.h

@@ -86,6 +86,13 @@ input_stream_open(const char *uri, GError **error_r);
 void
 input_stream_close(struct input_stream *is);
 
+/**
+ * Update the public attributes.  Call before accessing attributes
+ * such as "ready" or "offset".
+ */
+void
+input_stream_update(struct input_stream *is);
+
 /**
  * Seeks to the specified position in the stream.  This will most
  * likely fail if the "seekable" flag is false.