gst/gstbuffer.c: Copy duration/offset_end/caps when creating a subbuffer of the compl...

gst/gstbuffer.c: Copy duration/offset_end/caps when creating a subbuffer of the complete parent.

Original commit message from CVS:
* gst/gstbuffer.c: (gst_buffer_make_metadata_writable),
(gst_buffer_create_sub):
Copy duration/offset_end/caps when creating a subbuffer of the
complete parent.
Make the subbuffer read-only when we make the metadata writable for
now. Fixes #351768.
* tests/check/gst/gstbuffer.c: (GST_START_TEST):
Added check for metadata copy when creating subbuffers.