ffmpeg: treat avi as VFR in framerate conversion code

It can be debated if avi is vfr or cfr, it can be either depending
on the definition of vfr and cfr.
This commit ensures that avi is treated correctly independent of
the AVFMT_VARIABLE_FPS value.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -819,8 +819,12 @@ static void do_video_out(AVFormatContext *s,
     nb_frames = 1;
 
     format_video_sync = video_sync_method;
-    if (format_video_sync == VSYNC_AUTO)
-        format_video_sync = (s->oformat->flags & AVFMT_VARIABLE_FPS) ? ((s->oformat->flags & AVFMT_NOTIMESTAMPS) ? VSYNC_PASSTHROUGH : VSYNC_VFR) : VSYNC_CFR;
+    if (format_video_sync == VSYNC_AUTO) {
+        if(!strcmp(s->oformat->name, "avi")) {
+            format_video_sync = VSYNC_VFR;
+        } else
+            format_video_sync = (s->oformat->flags & AVFMT_VARIABLE_FPS) ? ((s->oformat->flags & AVFMT_NOTIMESTAMPS) ? VSYNC_PASSTHROUGH : VSYNC_VFR) : VSYNC_CFR;
+    }
 
     switch (format_video_sync) {
     case VSYNC_CFR: