Created attachment 255295 [details] [review]
Patch for custom clock slaving

The current audio sink slave methods do not offer a way to use available features for clock slaving, like adjustable PLLs, or external asynchronous resamplers. 

This patch introduces a new slave method, GST_AUDIO_BASE_SINK_SLAVE_CUSTOM. It delivers adjusted external and internal timestamps and discontinuity information to a user-defined callback. The callback can then do whatever it wants with this information, like controlling the aforementioned PLLs. The difference between external and internal timestamps is the clock skew, which the custom method can use as input value for its internal control loop.
The callback can also request the audiobasesink to skew the playout pointer by a defined number of nanoseconds. This is useful for large skews, which are difficult to overcome otherwise. For example, using asynchronous resampling only would require either drastic resampling, resulting in audible artifacts, or it would resample slightly, and it would take a long time for the skew to go away.
Discontinuities are reported to give the control loop the chance to clear internal accumulators/integrators.