fftools/ffmpeg: eliminate InputStream.got_output

It tracks whether the decoder for this stream ever produced any frames
and its only use is for checking whether a filter input ever received a
frame - those that did not are prioritized by the scheduler.

This is awkward and unnecessarily complicated - checking whether the
filtergraph input format is valid works just as well and does not
require maintaining an extra variable.

fftools/ffmpeg.c

@@ -1412,9 +1412,6 @@ static int process_input_packet(InputStream *ist, const AVPacket *pkt, int no_eo
             break;
         }
 
-        if (got_output)
-            ist->got_output = 1;
-
         if (!got_output)
             break;
 

fftools/ffmpeg.h

@@ -413,8 +413,6 @@ typedef struct InputStream {
     // number of frames/samples retrieved from the decoder
     uint64_t frames_decoded;
     uint64_t samples_decoded;
-
-    int got_output;
 } InputStream;
 
 typedef struct LastFrameDuration {

fftools/ffmpeg_filter.c

@@ -1700,7 +1700,7 @@ int fg_transcode_step(FilterGraph *graph, InputStream **best_ist)
         for (int i = 0; i < graph->nb_inputs; i++) {
             InputFilter *ifilter = graph->inputs[i];
             InputFilterPriv *ifp = ifp_from_ifilter(ifilter);
-            if (!ifp->ist->got_output && !ifp->eof) {
+            if (ifp->format < 0 && !ifp->eof) {
                 *best_ist = ifp->ist;
                 return 0;
             }