lavfi: add avfilter_get_class().

Useful for examining options, the same as the corresponding functions
for the other libs.

libavfilter/avfilter.c

@@ -693,3 +693,8 @@ int ff_filter_frame(AVFilterLink *link, AVFrame *frame)
 
     return filter_frame(link, out);
 }
+
+const AVClass *avfilter_get_class(void)
+{
+    return &avfilter_class;
+}

libavfilter/avfilter.h

@@ -690,4 +690,11 @@ attribute_deprecated
 int avfilter_copy_buf_props(AVFrame *dst, const AVFilterBufferRef *src);
 #endif
 
+/**
+ * @return AVClass for AVFilterContext.
+ *
+ * @see av_opt_find().
+ */
+const AVClass *avfilter_get_class(void);
+
 #endif /* AVFILTER_AVFILTER_H */