libs/gst/base/gstbasesrc.c: Don't update the last_stop position in do_seek, that...

libs/gst/base/gstbasesrc.c: Don't update the last_stop position in do_seek, that's the position we did a seek to.

Original commit message from CVS:
* libs/gst/base/gstbasesrc.c: (gst_base_src_default_do_seek),
(gst_base_src_loop):
Don't update the last_stop position in do_seek, that's the position we
did a seek to.
Read backwards when we have a negative rate.
* tests/check/elements/filesrc.c: (event_func), (wait_eos),
(setup_filesrc), (cleanup_filesrc), (GST_START_TEST),
(filesrc_suite):
Add check for reverse reading.