lavd probe fixes

Fixed incorrectly selected UYVY with following command:

    uv -t testcard:codec=R12L -c libavcodec:encoder=hevc_nvenc --param force-lavd-decoder=hevc_cuvid -d dummy -V

There were 2 problems:

1. probed codec should be returned after first successful decoded frame
   because in the above case, the decoder offers nv12 via get_frame
   callback first and it changes later to yuv444p16le if decoding
   continues and succeeds (see also 2.)

2. get_format_callback() returned VIDEO_CODDC_NONE on probe but that
   caused subsequent initialization of 'hevc' decoder (because lavd
   thought that it failed). Because of that, the probe couldn't have
   proceeded.

After the changes, the probed format is R10k (correct for now -
get_best_ug_codec_to_av({AV_PIX_FMT_YUV444P16, 0}, 0) returns R10k).

Author: MartinPulec

src/video_decompress/libavcodec.c

@@ -550,7 +550,7 @@ static enum AVPixelFormat get_format_callback(struct AVCodecContext *s, const en
                 codec_t c = get_best_ug_codec_to_av(fmt, hwaccel);
                 if (c != VIDEO_CODEC_NONE) {
                         state->internal_codec = c;
-                        return AV_PIX_FMT_NONE;
+                        return lavd_get_av_to_ug_codec(fmt, c, hwaccel);
                 }
         } else {
                 enum AVPixelFormat f = lavd_get_av_to_ug_codec(fmt, state->out_codec, hwaccel);
@@ -965,7 +965,7 @@ static decompress_status libavcodec_decompress(void *state, unsigned char *dst,
                 }
         }
 
-        if (s->out_codec == VIDEO_CODEC_NONE && s->internal_codec != VIDEO_CODEC_NONE) {
+        if (s->out_codec == VIDEO_CODEC_NONE && s->internal_codec != VIDEO_CODEC_NONE && res == DECODER_GOT_FRAME) {
                 *internal_codec = s->internal_codec;
                 return DECODER_GOT_CODEC;
         }