The clock source selection depends on priv->b_can_pace_control but this
variable is only initialized from the input_thread_t after the demux is
opened. Programs and ES tracks can be created from the demux open
callback or midstream (from the demux callback). Therefore, we can't
handle the clock source selection after the program is created since
priv->b_can_pace_control might not be initialized. To fix this issue,
handle clock source selection when the first PCR is sent (from
ES_OUT_SET_PCR).