fftools/ffmpeg: rework applying input -r

Do not use a separate counter for CFR timestamps forced with -r used as
an input option. Set durations properly and let estimation code do the
rest.

fftools/ffmpeg.h

@@ -372,10 +372,6 @@ typedef struct InputStream {
 
     int64_t filter_in_rescale_delta_last;
 
-    // when forcing constant input framerate through -r,
-    // this contains the pts that will be given to the next decoded frame
-    int64_t cfr_next_pts;
-
     int64_t nb_samples; /* number of samples in the last decoded audio frame before looping */
 
     AVDictionary *decoder_opts;