gst/gstbuffer.c: Set READONLY flag on subbuffers, so that gst_buffer_make_writable() works correctly in all circumsta...

Original commit message from CVS:
* gst/gstbuffer.c: (gst_subbuffer_init):
Set READONLY flag on subbuffers, so that gst_buffer_make_writable()
works correctly in all circumstances (we could have just copied
the parent buffer's readonly flag, but conceptually it seems
cleaner to mark all subbuffers as read-only). (based on patch
by Alessandro Decina, #314710).
* check/gst/gstbuffer.c: (create_read_only_buffer),
(test_make_writable), (test_subbuffer_make_writable),
(gst_test_suite):
Add some tests for gst_buffer_make_writable().