Allow basesrc derived classes to execute seeks in other formats by providing a prepare_seek_segment vmethod. Sub-clas...

Original commit message from CVS:
* docs/libs/gstreamer-libs-sections.txt:
* libs/gst/base/gstbasesrc.c: (gst_base_src_class_init),
(gst_base_src_default_prepare_seek_segment),
(gst_base_src_prepare_seek_segment), (gst_base_src_perform_seek):
* libs/gst/base/gstbasesrc.h:
Allow basesrc derived classes to execute seeks in other formats
by providing a prepare_seek_segment vmethod. Sub-classes can choose
to prepare the GstSegment in any format that their perform_seek method
will be able to understand. The default implementation provides the
old behaviour of attempting to convert the seek offsets to the
configured native format.