libs/gst/base/gstbasesink.h: New GstBaseSinkClass vmethod, activate_pull(), providing for a way to specialize the pro...

Original commit message from CVS:
2007-01-06  Andy Wingo  <wingo@pobox.com>

* libs/gst/base/gstbasesink.h: New GstBaseSinkClass vmethod,
activate_pull(), providing for a way to specialize the process of
spawning a thread to pull on the sink pad. There is a default
implementation.

* libs/gst/base/gstbasesink.c (gst_base_sink_pad_activate_pull)
(gst_base_sink_pad_activate_push, gst_base_sink_pad_activate)
(gst_base_sink_init): Renamed pad activation functions (inserting
"_pad" in their names). Refactor to use the new activate_pull
vmethod, as appropriate.
(gst_base_sink_class_init, gst_base_sink_activate_pull): Set the
default activate_pull function to start a task pulling from the
sink pad, as before.