gst/gstpad.c: implement enforcing discont events before buffers are passed. This...

gst/gstpad.c: implement enforcing discont events before buffers are passed. This allows state changes of only some el...

Original commit message from CVS:
* gst/gstpad.c: (gst_pad_set_active), (gst_pad_link_new),
(gst_pad_link_free), (gst_pad_link_try), (_invent_event),
(gst_pad_pull):
implement enforcing discont events before buffers are passed. This
allows state changes of only some elements and later correctly going
on where they left off (or in short: you can now set audio sinks to
NULL to release the device when the pipeline is paused)
* gst/gstpad.c: (gst_pad_call_chain_function),
(gst_pad_call_get_function):
* gst/gstpad.h:
add gst_pad_call_chain_function and gst_pad_call_get_function for
scheduler interaction. They are required because of the changes
above.
* gst/schedulers/entryscheduler.c: (get_buffer),
(gst_entry_scheduler_chain_wrapper),
(gst_entry_scheduler_get_wrapper),
(gst_entry_scheduler_state_transition),
(gst_entry_scheduler_pad_link):
* gst/schedulers/gstbasicscheduler.c:
(gst_basic_scheduler_chain_wrapper),
(gst_basic_scheduler_src_wrapper),
(gst_basic_scheduler_chainhandler_proxy),
(gst_basic_scheduler_gethandler_proxy),
(gst_basic_scheduler_cothreaded_chain),
(gst_basic_scheduler_chain_elements):
* gst/schedulers/gstoptimalscheduler.c:
(get_group_schedule_function), (pad_clear_queued),
(gst_opt_scheduler_pad_link):
use the new functions instead of calling get/chain-functions
directly.