No reason to not enable threads if lavc supports them, same as for
software decoding. Might help with pipelining of decoding and filtering.
Performance difference may vary, but in my testing everything is working
as expected and I didn't notice any regressions.
Set to 2 threads by default as conservative value that should resolve
stalling issue, while also not spawn too many cpu threads, which won't
help with hwdec anyway and only can cause too deep queue of hardware
surfaces.
Fixes: #16578
dbda70a11c