get_ug_codec_to_av: prefer first AV codecs

If earlier codec is convertible, use this. In other words, prefer first
codecs, not those most easily convertible.

The point is, that the list passed to get_format lavd callback is
usually ordered from best codecs so we want to keep it.

It also fixes a "regression" from 4c5cf6f (given by the different
selection schemes in original and the replacement):

   uv -t testcard:codec=R12L -c libavcodec:encoder=libsvt_hevc -d dummy \
        --param force-lavd-decoder=libde265 -V

decoding YUYV, not v210 (from AVPixelFomat AV_PIX_FMT_YUV444P10LE)

Author: MartinPulec

src/libavcodec/from_lavc_vid_conv.c

@@ -1982,8 +1982,9 @@ av_to_uv_convert_p get_av_to_uv_conversion(int av_codec, codec_t uv_codec) {
 }
 
 /**
- * Returns AVPixelFormat matching *ugc. If !*ugc, finds (probes) best UltraGrid codec
- * to which can be one of fmt converted and returns AV_PIX_FMT_NONE.
+ * Returns first AVPixelFormat convertible to *ugc. If !*ugc, finds (probes)
+ * best UltraGrid codec to which can be one of fmt converted and returns
+ * AV_PIX_FMT_NONE.
  *
  * @param[in,out] ugc        if zero, probing the codec, if nonzero, only finding matching AVPixelFormat
  * @retval AV_PIX_FMT_NONE   if !*ugc
@@ -2002,11 +2003,9 @@ static enum AVPixelFormat get_ug_codec_to_av(const enum AVPixelFormat *fmt, code
                                 return *fmt_it;
                         }
                 }
-        }
-        bool use_native[] = { true, false }; // try native first
-        for (const bool *use_native_it = use_native; use_native_it !=
-                        use_native + sizeof use_native / sizeof use_native[0]; ++use_native_it) {
-                for (const enum AVPixelFormat *fmt_it = fmt; *fmt_it != AV_PIX_FMT_NONE; fmt_it++) {
+                bool use_native[] = { true, false }; // try native first
+                for (const bool *use_native_it = use_native; use_native_it !=
+                                use_native + sizeof use_native / sizeof use_native[0]; ++use_native_it) {
                         //If hwaccel is not enabled skip hw accel pixfmts even if there
                         //are convert functions
                         const AVPixFmtDescriptor *fmt_desc = av_pix_fmt_desc_get(*fmt_it);