swscale: set thread count to auto

This enables multithreaded swscale conversion newly added to FFmpeg 5.

Author: MartinPulec

src/libavcodec_common.c

@@ -2939,6 +2939,7 @@ struct SwsContext *getSwsContext(unsigned int SrcW, unsigned int SrcH, enum AVPi
     int SrcRange = SrcFormatDesc != NULL && (SrcFormatDesc->flags & AV_PIX_FMT_FLAG_RGB) != 0 ? 1 : 0;
     int DstRange = DstFormatDesc != NULL && (DstFormatDesc->flags & AV_PIX_FMT_FLAG_RGB) != 0 ? 1 : 0;
 
+    int rc = 0;
     av_opt_set_int(Context, "sws_flags",  Flags, 0);
     av_opt_set_int(Context, "srcw",       SrcW, 0);
     av_opt_set_int(Context, "srch",       SrcH, 0);
@@ -2948,6 +2949,9 @@ struct SwsContext *getSwsContext(unsigned int SrcW, unsigned int SrcH, enum AVPi
     av_opt_set_int(Context, "dst_range",  DstRange, 0);
     av_opt_set_int(Context, "src_format", SrcFormat, 0);
     av_opt_set_int(Context, "dst_format", DstFormat, 0);
+    if ((rc = av_opt_set(Context, "thread", "auto", 0)) != 0) {
+            print_libav_error(LOG_LEVEL_VERBOSE, "Swscale - cannot set thread mode", rc);
+    }
 
     if (sws_init_context(Context, 0, 0) < 0) {
         sws_freeContext(Context);