avcodec: refactor frame_rate probing into own function

if codec has list of supported framerates, select highest one
unless user has provided one. If we end up using CLOCK_FREQ in
time_base, it should be quite ok in most cases. Tested with ts/mp4/webm
and mp2v/mp4v/VP80. For some cases it can select higher fps in
mpeg-codecs (60fps) in case of input fps isn't yet known in that point
or is fubared.

Also add special case for MP4V as it doesn't like CLOCK_FREQ in
time_base/frame_rates.

modules/codec/avcodec/encoder.c

@@ -238,6 +238,44 @@ static const int DEFAULT_ALIGN = 0;
 /*****************************************************************************
  * OpenEncoder: probe the encoder
  *****************************************************************************/
+static void probe_video_frame_rate( encoder_t *p_enc, AVCodecContext *p_context, AVCodec *p_codec )
+{
+    /* if we don't have i_frame_rate_base, we are probing and just checking if we can find codec
+     * so set fps to requested fps if asked by user or input fps is availabled */
+    p_context->time_base.num = p_enc->fmt_in.video.i_frame_rate_base ? p_enc->fmt_in.video.i_frame_rate_base : 1;
+
+    // MP4V doesn't like CLOCK_FREQ denominator in time_base, so use 1/25 as default for that
+    p_context->time_base.den = p_enc->fmt_in.video.i_frame_rate_base ? p_enc->fmt_in.video.i_frame_rate :
+                                  ( p_enc->fmt_out.i_codec == VLC_CODEC_MP4V ? 25 : CLOCK_FREQ );
+
+    msg_Dbg( p_enc, "Time base for probing setted to %d/%d", p_context->time_base.num, p_context->time_base.den );
+    if( p_codec->supported_framerates )
+    {
+        /* We are finding fps values so 1/time_base */
+        AVRational target = {
+            .num = p_context->time_base.den,
+            .den = p_context->time_base.num
+        };
+        int idx = av_find_nearest_q_idx(target, p_codec->supported_framerates);
+
+        p_context->time_base.num = p_codec->supported_framerates[idx].den ?
+                                    p_codec->supported_framerates[idx].den : 1;
+        p_context->time_base.den = p_codec->supported_framerates[idx].den ?
+                                    p_codec->supported_framerates[idx].num : CLOCK_FREQ;
+
+        /* If we have something reasonable on supported framerates, use that*/
+        if( p_context->time_base.den && p_context->time_base.den < CLOCK_FREQ )
+        {
+            p_enc->fmt_out.video.i_frame_rate_base =
+                p_enc->fmt_in.video.i_frame_rate_base =
+                p_context->time_base.num;
+            p_enc->fmt_out.video.i_frame_rate =
+                p_enc->fmt_in.video.i_frame_rate =
+                p_context->time_base.den;
+        }
+    }
+    msg_Dbg( p_enc, "Time base set to %d/%d", p_context->time_base.num, p_context->time_base.den );
+}
 
 int OpenEncoder( vlc_object_t *p_this )
 {
@@ -463,22 +501,7 @@ int OpenEncoder( vlc_object_t *p_this )
         p_context->width = p_enc->fmt_in.video.i_visible_width;
         p_context->height = p_enc->fmt_in.video.i_visible_height;
 
-        /* if we don't have i_frame_rate_base, we are probing and just checking if we can find codec
-         * so set fps to 25 as some codecs (DIV3 atleast) needs time_base data */
-        p_context->time_base.num = p_enc->fmt_in.video.i_frame_rate_base ? p_enc->fmt_in.video.i_frame_rate_base : 1;
-        p_context->time_base.den = p_enc->fmt_in.video.i_frame_rate_base ? p_enc->fmt_in.video.i_frame_rate : 25 ;
-        if( p_codec->supported_framerates )
-        {
-            AVRational target = {
-                .num = p_enc->fmt_in.video.i_frame_rate,
-                .den = p_enc->fmt_in.video.i_frame_rate_base,
-            };
-            int idx = av_find_nearest_q_idx(target, p_codec->supported_framerates);
-
-            p_context->time_base.num = p_codec->supported_framerates[idx].den;
-            p_context->time_base.den = p_codec->supported_framerates[idx].num;
-        }
-        msg_Dbg( p_enc, "Time base set to %d/%d", p_context->time_base.num, p_context->time_base.den );
+        probe_video_frame_rate( p_enc, p_context, p_codec );
 
         /* Defaults from ffmpeg.c */
         p_context->qblur = 0.5;