Pass AVCodecContext to packet sinks

Create the codec context from the demuxer, so that it can fill context
data for the decoder and recorder.

app/src/decoder.h

@@ -16,7 +16,7 @@ struct sc_decoder {
 
     const char *name; // must be statically allocated (e.g. a string literal)
 
-    AVCodecContext *codec_ctx;
+    AVCodecContext *ctx;
     AVFrame *frame;
 };