libs/gst/base/gstbasesink.*: Add function to wait for EOS, subclasses can use this...

libs/gst/base/gstbasesink.*: Add function to wait for EOS, subclasses can use this to correctly wait for devices to d...

Original commit message from CVS:
Patch inspired by: Benoit Fouet <benoit dot fouet at purplelabs dot com>
* libs/gst/base/gstbasesink.c: (gst_base_sink_wait_eos),
(gst_base_sink_event):
* libs/gst/base/gstbasesink.h:
Add function to wait for EOS, subclasses can use this to correctly wait
for devices to drain before performing the EOS logic. Fixes #485343.
API: gst_base_sink_wait_eos()