Be notified of different states of pads.
The provided callback is called for every state that matches
Probes are called in groups: First GST_PAD_PROBE_TYPE_BLOCK probes are called, then others, then finally GST_PAD_PROBE_TYPE_IDLE. The only exception here are GST_PAD_PROBE_TYPE_IDLE probes that are called immediately if the pad is already idle while calling add_probe. In each of the groups, probes are called in the order in which they were added.
the Pad to add the probe to
the probe mask
PadProbeCallback that will be called with notifications of the pad state
DestroyNotify for user_data
user data passed to the callback