lavc/libx264: Use avctx->framerate first for frame rate setting

perfer avctx->framerate first than use avctx->time_base when setting
the frame rate to encoder. 1/time_base is not the average frame rate
if the frame rate is not constant. In this case, we need to setting
avctx->framerate and avctx->time_base both, but avctx->framerate not
equal to 1/(avctx->time_base).
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavcodec/libx264.c

@@ -821,8 +821,13 @@ FF_ENABLE_DEPRECATION_WARNINGS
     x4->params.vui.i_sar_height = sh;
     x4->params.i_timebase_den = avctx->time_base.den;
     x4->params.i_timebase_num = avctx->time_base.num;
-    x4->params.i_fps_num = avctx->time_base.den;
-    x4->params.i_fps_den = avctx->time_base.num * avctx->ticks_per_frame;
+    if (avctx->framerate.num > 0 && avctx->framerate.den > 0) {
+        x4->params.i_fps_num = avctx->framerate.num;
+        x4->params.i_fps_den = avctx->framerate.den;
+    } else {
+        x4->params.i_fps_num = avctx->time_base.den;
+        x4->params.i_fps_den = avctx->time_base.num * avctx->ticks_per_frame;
+    }
 
     x4->params.analyse.b_psnr = avctx->flags & AV_CODEC_FLAG_PSNR;