fftools/ffmpeg: rework -enc_time_base handling

Read the timebase from FrameData rather than the input stream. This
should fix #10393 and generally be more reliable.

Replace the use of '-1' to indicate demuxing timebase with the string
'demux'. Also allow to request filter timebase with
'-enc_time_base filter'.

fftools/ffmpeg.h

@@ -56,6 +56,7 @@
 #define FFMPEG_ROTATION_METADATA 1
 #define FFMPEG_OPT_QPHIST 1
 #define FFMPEG_OPT_ADRIFT_THRESHOLD 1
+#define FFMPEG_OPT_ENC_TIME_BASE_NUM 1
 
 enum VideoSyncMethod {
     VSYNC_AUTO = -1,
@@ -66,6 +67,11 @@ enum VideoSyncMethod {
     VSYNC_DROP,
 };
 
+enum EncTimeBase {
+    ENC_TIME_BASE_DEMUX  = -1,
+    ENC_TIME_BASE_FILTER = -2,
+};
+
 #define MAX_STREAMS 1024    /* arbitrary sanity check value */
 
 enum HWAccelID {