The libvpx doxy says that a value of 0 for the g_threads field is
equivalent to a value of 1, whereas for avctx->thread_count it means
the maximum amount of threads possible for the host system.

Use av_cpu_count() to get the correct thread count when auto threads
is requested.
Reviewed-by: James Zern <jzern@google.com>
Signed-off-by: James Almer <jamrial@gmail.com>