fixed lavd always selected

Fixes the commit b62566c (2th Oct 2023), which caused that lavd was
always selected as the decoder, even though not designated. Eg.:
```
$ uv -t testcard:codec=I420 -d gl
Decompressor "libavcodec" initialized successfully.
[lavd] Unsupported codec!!!
```

The problem was that on probe ugc==VC_NONE and HW acceleration is not
set, thus hw_accel_to_ug_pixfmt(0) return also VC_NONE, in which case
the decoder is unconditionally selected for all formats.

Author: MartinPulec

src/video_decompress/libavcodec.c

@@ -1156,7 +1156,8 @@ static int libavcodec_decompress_get_priority(codec_t compression, struct pixfmt
                 PRIO_LOWER = 800,
                 PRIO_LO    = 900,
         };
-        if (hw_accel_to_ug_pixfmt(internal.accel_type) == ugc) {
+        if (internal.accel_type != HWACCEL_NONE &&
+            hw_accel_to_ug_pixfmt(internal.accel_type) == ugc) {
                 return PRIO_HI;
         }