es_out: fix VLC_CLOCK_MASTER_AUTO detection
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).