fftools/ffmpeg_filter: pass framerate through InputFilterOptions

Rather than read it directly from InputStream.

This is a step towards avoiding the assumption that filtergraph inputs
are always fed by demuxers.

fftools/ffmpeg.h

@@ -253,6 +253,7 @@ typedef struct OptionsContext {
 enum IFilterFlags {
     IFILTER_FLAG_AUTOROTATE     = (1 << 0),
     IFILTER_FLAG_REINIT         = (1 << 1),
+    IFILTER_FLAG_CFR            = (1 << 2),
 };
 
 typedef struct InputFilterOptions {
@@ -261,6 +262,13 @@ typedef struct InputFilterOptions {
 
     uint8_t            *name;
 
+    /* When IFILTER_FLAG_CFR is set, the stream is guaranteed to be CFR with
+     * this framerate.
+     *
+     * Otherwise, this is an estimate that should not be relied upon to be
+     * accurate */
+    AVRational          framerate;
+
     int                 sub2video_width;
     int                 sub2video_height;
 
@@ -365,8 +373,6 @@ typedef struct InputStream {
     Decoder              *decoder;
     const AVCodec        *dec;
 
-    AVRational            framerate_guessed;
-
     /* framerate forced with -r */
     AVRational            framerate;
 #if FFMPEG_OPT_TOP