Don't try to set port attribute that's not advertised by the
adaptor. Fixes videotestsrc ! xvimagesink aborting with
X Error of failed request: BadMatch (invalid parameter attributes)
Major opcode of failed request: 151 (XVideo)
Minor opcode of failed request: 13 ()
on intel HD4600 graphics with kernel 3.16, xserver 1.15,
intel driver 2.21.15.
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.
