Used AVCodecContext::ticks_per_frame when available.

It fixes a regression with interpolated timestamps for some
mpeg-I/II and for h264.
(cherry picked from commit 06105bd1)
Signed-off-by: Derk-Jan Hartman <hartman@videolan.org>

modules/codec/avcodec/video.c

@@ -1046,9 +1046,17 @@ static void ffmpeg_NextPts( decoder_t *p_dec )
     }
     else if( p_sys->p_context->time_base.den > 0 )
     {
+#if LIBAVCODEC_VERSION_INT >= AV_VERSION_INT(52,20,0)
+        int i_tick = i_tick = p_sys->p_context->ticks_per_frame;
+        if( i_tick <= 0 )
+            i_tick = 1;
+#else
+        int i_tick = 1;
+#endif
+
         p_sys->i_pts += INT64_C(1000000) *
             (2 + p_sys->p_ff_pic->repeat_pict) *
-            p_sys->p_context->time_base.num /
+            i_tick * p_sys->p_context->time_base.num /
             (2 * p_sys->p_context->time_base.den);
     }
 }