gst/flacparse/gstbaseparse.*: Provide a vfunc for the subclass to decide whether...

gst/flacparse/gstbaseparse.*: Provide a vfunc for the subclass to decide whether a frame is inside the segment or not...

Original commit message from CVS:
* gst/flacparse/gstbaseparse.c: (gst_base_parse_class_init),
(gst_base_parse_push_buffer),
(gst_base_parse_update_upstream_durations),
(gst_base_parse_convert), (gst_base_parse_frame_in_segment):
* gst/flacparse/gstbaseparse.h:
Provide a vfunc for the subclass to decide whether a frame is inside
the segment or not and add a default implementation.
Fix approximate bitrate calculations.