Adds function image_copy_ref to image module for getting a new reference
to an image. It can be used instead of image_make_subimage when the
sizes of the original and the subimage are same.
- Make sure that everything which is allocated gets deallocated.
- Move finalization of encoder states to kvazaar.c.
- Remove empty strategyselector_free function.
- Remove unused variable curpos.
- Fix includes.
- Use the existing bitstream_t type to give access to the bitstream.
We can extend it later to make it a linked list like I was planning
to do with the payload type.
- The main encoder now also stores the bitstream in memory.
