vcomp/lavc: thr_cnt=0 for other threads implicitly

Set automatic number (== 0) of threads if codec advertises
AV_CODEC_CAP_OTHER_THREAD implicitily, with exceptions:
1. AOM AV1, rav1e - seems not to set the number automatically
2. libx26* - allow setting but enables frame threading internally
(which is undesirable)

the current setting applies not just to libvpx* but also to
eg. libopenh264.

src/video_compress/libavcodec.cpp

@@ -1562,11 +1562,12 @@ static void set_codec_thread_mode(AVCodecContext *codec_ctx, struct setparam_par
                 codec_ctx->thread_count = req_thread_count;
         } else if ((codec_ctx->codec->capabilities & AV_CODEC_CAP_OTHER_THREADS) != 0) {
                 // do not enable MT for eg. libx265 - libx265 uses frame threads
-                if (strncmp(codec_ctx->codec->name, "libvpx", 6) == 0) {
-                        codec_ctx->thread_count = 0;
-                } else if (strcmp(codec_ctx->codec->name, "libaom-av1") == 0 ||
+                if (strcmp(codec_ctx->codec->name, "libaom-av1") == 0 ||
                                 strcmp(codec_ctx->codec->name, "librav1e") == 0) {
                         codec_ctx->thread_count = thread::hardware_concurrency();
+                } else if (strstr(codec_ctx->codec->name, "libx26") !=
+                           codec_ctx->codec->name) {
+                        codec_ctx->thread_count = 0;
                 }
         } else if (codec_ctx->thread_type != 0) {
                 codec_ctx->thread_count = thread::hardware_concurrency();