lavc: do not enable MT with CAP_OTHER_THREADS

Only enable for libvpx*.

Codecs must be individually evaluated - for libx265, it increases
latency because it uses frame threading parallelism internally.

For libsvt_hevc and libx264, the performance boost is not noticeable.

Author: MartinPulec

src/video_compress/libavcodec.cpp

@@ -1692,8 +1692,10 @@ static void set_codec_thread_mode(AVCodecContext *codec_ctx, struct setparam_par
         if (req_thread_count != -1) {
                 codec_ctx->thread_count = req_thread_count;
         } else if ((codec_ctx->codec->capabilities & AV_CODEC_CAP_OTHER_THREADS) != 0) {
-                // mainly for libvpx-vp9, libx264 has thread_count set implicitly to 0 (auto)
-                codec_ctx->thread_count = 0;
+                // do not enable MT for eg. libx265 - libx265 uses frame threads
+                if (strncmp(codec_ctx->codec->name, "libvpx", 6)) {
+                        codec_ctx->thread_count = 0;
+                }
         } else if (codec_ctx->thread_type != 0) {
                 codec_ctx->thread_count = thread::hardware_concurrency();
         }