pixdesc: Improve scoring for opaque/unknown pixel formats

Hardware pixel formats do not tell you anything about their actual
contents, but should still score higher than formats with completely
unknown properties, which in turn should score higher than invalid
formats.

Do not return an AVERROR code as a score.

Fixes a hang in libavfilter where format negotiation gets stuck in a
loop because AV_PIX_FMT_NONE scores more highly than all other
possibilities.

libavutil/pixdesc.c

@@ -2511,8 +2511,16 @@ static int get_pix_fmt_score(enum AVPixelFormat dst_pix_fmt,
     int ret, loss, i, nb_components;
     int score = INT_MAX - 1;
 
-    if (dst_pix_fmt >= AV_PIX_FMT_NB || dst_pix_fmt <= AV_PIX_FMT_NONE)
-        return ~0;
+    if (!src_desc || !dst_desc)
+        return -4;
+
+    if ((src_desc->flags & AV_PIX_FMT_FLAG_HWACCEL) ||
+        (dst_desc->flags & AV_PIX_FMT_FLAG_HWACCEL)) {
+        if (dst_pix_fmt == src_pix_fmt)
+            return -1;
+        else
+            return -2;
+    }
 
     /* compute loss */
     *lossp = loss = 0;
@@ -2521,9 +2529,9 @@ static int get_pix_fmt_score(enum AVPixelFormat dst_pix_fmt,
         return INT_MAX;
 
     if ((ret = get_pix_fmt_depth(&src_min_depth, &src_max_depth, src_pix_fmt)) < 0)
-        return ret;
+        return -3;
     if ((ret = get_pix_fmt_depth(&dst_min_depth, &dst_max_depth, dst_pix_fmt)) < 0)
-        return ret;
+        return -3;
 
     src_color = get_color_type(src_desc);
     dst_color = get_color_type(dst_desc);