fftools/ffmpeg_filter: add an AVClass to OutputFilter

Use it for logging where appropriate, avoid logging to OutputStream as
we do not own it.

This is a step towards decoupling filtering from encoding.

fftools/ffmpeg.h

@@ -302,6 +302,8 @@ typedef struct InputFilter {
 } InputFilter;
 
 typedef struct OutputFilter {
+    const AVClass       *class;
+
     struct OutputStream *ost;
     struct FilterGraph  *graph;
     uint8_t             *name;