libvpx: Set the libvpx frame dropping threshold to frame_skip_threshold

The default for frame_skip_threshold is 0, which thus disables the libvpx
frame dropping.

Originally committed as revision 23522 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/libvpxenc.c

@@ -240,6 +240,7 @@ static av_cold int vp8_init(AVCodecContext *avctx)
     //convert [1,51] -> [0,63]
     enccfg.rc_min_quantizer = ((avctx->qmin * 5 + 1) >> 2) - 1;
     enccfg.rc_max_quantizer = ((avctx->qmax * 5 + 1) >> 2) - 1;
+    enccfg.rc_dropframe_thresh = avctx->frame_skip_threshold;
 
     //_enc_init() will balk if kf_min_dist differs from max w/VPX_KF_AUTO
     if (avctx->keyint_min == avctx->gop_size)