swr: add swr_get_class()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/swresample.h

@@ -54,6 +54,14 @@ enum SwrDitherType {
 
 typedef struct SwrContext SwrContext;
 
+/**
+ * Get the AVClass for swrContext. It can be used in combination with
+ * AV_OPT_SEARCH_FAKE_OBJ for examining options.
+ *
+ * @see av_opt_find().
+ */
+const AVClass *swr_get_class(void);
+
 /**
  * Allocate SwrContext.
  *