Fix build for FFmpeg < 3.3

The constant AV_CODEC_ID_AV1 was introduced in FFmpeg 3.3. Add an ifdef
to support older versions.

Fixes #3939 <https://github.com/Genymobile/scrcpy/issues/3939>

app/src/compat.h

@@ -25,6 +25,12 @@
 # define SCRCPY_LAVF_REQUIRES_REGISTER_ALL
 #endif
 
+// Not documented in ffmpeg/doc/APIchanges, but AV_CODEC_ID_AV1 has been added
+// by FFmpeg commit d42809f9835a4e9e5c7c63210abb09ad0ef19cfb (included in tag
+// n3.3).
+#if LIBAVFORMAT_VERSION_INT >= AV_VERSION_INT(57, 89, 100)
+# define SCRCPY_LAVC_HAS_AV1
+#endif
 
 // In ffmpeg/doc/APIchanges:
 // 2018-01-28 - ea3672b7d6 - lavf 58.7.100 - avformat.h

app/src/demuxer.c

@@ -33,7 +33,12 @@ sc_demuxer_to_avcodec_id(uint32_t codec_id) {
         case SC_CODEC_ID_H265:
             return AV_CODEC_ID_HEVC;
         case SC_CODEC_ID_AV1:
+#ifdef SCRCPY_LAVC_HAS_AV1
             return AV_CODEC_ID_AV1;
+#else
+            LOGE("AV1 not supported by this FFmpeg version");
+            return AV_CODEC_ID_NONE;
+#endif
         case SC_CODEC_ID_OPUS:
             return AV_CODEC_ID_OPUS;
         case SC_CODEC_ID_AAC: