avfilter/vf_uspp: Add grayscale support

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

libavfilter/vf_uspp.c

@@ -294,6 +294,7 @@ static int query_formats(AVFilterContext *ctx)
         AV_PIX_FMT_YUV410P,
         AV_PIX_FMT_YUVJ444P,
         AV_PIX_FMT_YUVJ420P,
+        AV_PIX_FMT_GRAY8,
         AV_PIX_FMT_NONE
     };
     ff_set_common_formats(ctx, ff_make_format_list(pix_fmts));