vf_fieldorder: avoid using AV_PIX_FMT_NB

That hardcodes the number of pixel formats into lavfi and will break
when a shared lavu is updated, adding new pixel formats.

libavfilter/vf_fieldorder.c

@@ -50,9 +50,10 @@ static int query_formats(AVFilterContext *ctx)
     /** accept any input pixel format that is not hardware accelerated, not
      *  a bitstream format, and does not have vertically sub-sampled chroma */
     if (ctx->inputs[0]) {
+        const AVPixFmtDescriptor *desc = NULL;
         formats = NULL;
-        for (pix_fmt = 0; pix_fmt < AV_PIX_FMT_NB; pix_fmt++) {
+        while ((desc = av_pix_fmt_desc_next(desc))) {
-            const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pix_fmt);
+            pix_fmt = av_pix_fmt_desc_get_id(desc);
             if (!(desc->flags & AV_PIX_FMT_FLAG_HWACCEL ||
                   desc->flags & AV_PIX_FMT_FLAG_BITSTREAM) &&
                 desc->nb_components && !desc->log2_chroma_h &&