Limit g_threads to MAX_NUM_THREADS (64)

[mayli]

On system with more than 256 cpus, both aom and vpx encoder will fail with

aom
        Failed to initialize encoder: Invalid parameter
            g_threads out of range [..MAX_NUM_THREADS]
vpx
        Failed to initialize encoder: Invalid parameter
            g_threads out of range [..64]

btw, -v '--threads=32' doesn't seem to work.