avfilter/vf_uspp: fix used pix_fmt

Found-by: carl
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vf_uspp.c

@@ -342,7 +342,7 @@ static int config_input(AVFilterLink *inlink)
         avctx_enc->time_base = (AVRational){1,25};  // meaningless
         avctx_enc->gop_size = 300;
         avctx_enc->max_b_frames = 0;
-        avctx_enc->pix_fmt = AV_PIX_FMT_YUV420P;
+        avctx_enc->pix_fmt = inlink->format;
         avctx_enc->flags = CODEC_FLAG_QSCALE | CODEC_FLAG_LOW_DELAY;
         avctx_enc->strict_std_compliance = FF_COMPLIANCE_EXPERIMENTAL;
         avctx_enc->global_quality = 123;