vpxenc: add VP8E_SET_STATIC_THRESHOLD mapping

via the equivalent AVCodecContext::mb_threshold
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libvpxenc.c

@@ -311,6 +311,7 @@ static av_cold int vp8_init(AVCodecContext *avctx)
     codecctl_int(avctx, VP8E_SET_CPUUSED,           cpuused);
     codecctl_int(avctx, VP8E_SET_NOISE_SENSITIVITY, avctx->noise_reduction);
     codecctl_int(avctx, VP8E_SET_TOKEN_PARTITIONS,  av_log2(avctx->slices));
+    codecctl_int(avctx, VP8E_SET_STATIC_THRESHOLD,  avctx->mb_threshold);
 
     //provide dummy value to initialize wrapper, values will be updated each _encode()
     vpx_img_wrap(&ctx->rawimg, VPX_IMG_FMT_I420, avctx->width, avctx->height, 1,