xvimagesink: use xvcontext for allocation

xvimagesink: use xvcontext for allocation

Make a new refcounted xvcontext object that handles the X connection.
Use the xvcontext to allocate images and windows. Move some code
around so that all X calls are made from the xvcontext object.
Make a GstXvImageAllocator object that allocates images from the xvcontext. We
can implement a copy function now for these memory objects now.
Make the bufferpool use the xvimageallocator object for its images.