Add GStreamer::Buffer::data_ptr()

It enables access to the buffer's data without copying it.  Based on a patch by
Egor Korablev.

https://rt.cpan.org/Ticket/Display.html?id=82826
Ignore MYMETA.*
Updated copyright date
Converted Makefile.PL to use CPAN Meta Spec

- Created %meta_merge which follows v2 of meta-spec
- Renamed %pre_reqs to $PREREQ_PM in Makefile.PL, which is what's used
  in most other Gtk2-Perl modules
Stable release 0.18
Avoid misusing PL_na