FFmpeg: Fix off by one error in number of detected frames in matroska container

Seems to be rounding error. Hopefully new code handles the error fixed back in
SVN revision 28901 and still have proper frame number for Hjalti.

What could possibly go wrong here..

source/blender/imbuf/intern/anim_movie.c

@@ -522,9 +522,9 @@ static int startffmpeg(struct anim *anim)
 		anim->duration = pFormatCtx->streams[videoStream]->nb_frames;
 	}
 	else {
-		anim->duration = ceil(pFormatCtx->duration *
-		                      av_q2d(frame_rate) /
-		                      AV_TIME_BASE);
+		anim->duration = (int)(pFormatCtx->duration *
+		                       av_q2d(frame_rate) /
+		                       AV_TIME_BASE + 0.5f);
 	}
 
 	frs_num = frame_rate.num;