setpts: add FRAME_RATE constant

Useful for dealing with constant frame-rate video.

(cherry picked from ffmpeg commit 722762f7)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/filters.texi

@@ -209,6 +209,9 @@ The expression is evaluated through the eval API and can contain the following
 constants:
 
 @table @option
+@item FRAME_RATE
+frame rate, only defined for constant frame-rate video
+
 @item PTS
 the presentation timestamp in input
 

libavfilter/setpts.c

@@ -41,6 +41,7 @@
 
 static const char *const var_names[] = {
     "E",           ///< Euler number
+    "FRAME_RATE",  ///< defined only for constant frame-rate video
     "INTERLACED",  ///< tell if the current frame is interlaced
     "N",           ///< frame / sample number (starting at zero)
     "PHI",         ///< golden ratio
@@ -59,6 +60,7 @@ static const char *const var_names[] = {
 
 enum var_name {
     VAR_E,
+    VAR_FRAME_RATE,
     VAR_INTERLACED,
     VAR_N,
     VAR_PHI,
@@ -115,6 +117,13 @@ static int config_input(AVFilterLink *inlink)
         setpts->var_values[VAR_SR] = inlink->sample_rate;
     }
 
+    setpts->var_values[VAR_FRAME_RATE] = inlink->frame_rate.num &&
+                                         inlink->frame_rate.den ?
+                                            av_q2d(inlink->frame_rate) : NAN;
+
+    // Indicate the output can be variable framerate.
+    inlink->frame_rate = (AVRational){1, 0};
+
     av_log(inlink->src, AV_LOG_VERBOSE, "TB:%f\n", setpts->var_values[VAR_TB]);
     return 0;
 }