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>