[kwin_wayland] Add support for shm buffers in server module

The Display provides a method to create the shm pool and a
BufferInterface class is added to the server module. It is created
from the SurfaceInterface when a buffer gets attached to the surface.
The BufferInterface can be referenced and once its unreferenced it
sends a buffer release to the client and destroys itself.

For the case that the buffer is a shm buffer the BufferInterface
provides a convenience method to turn it into a QImage.

The auto test for Surface is extended by attaching buffers to the
surface and verifying that the content is correct.