Provide an easy way to monitor buffer size changes

The main purpose behind the kwaylandserver library is to provide a set
of re-usable wayland compositor extension implementations. However, it's
worth noting that the design of kwaylandserver is far from perfect at
the moment.

KWaylandServer tries to hide all low level wayland details from the
compositor. But it's not the case with buffers, which diminishes the
whole point behind the library.

Creating OpenGL textures from Wayland buffers is the responsibility of
the compositor. So, when it comes to client buffers, we are one foot in
KWaylandServer, and the other foot in the compositor.

Since the surface size is a logical size, the compositor can't use it
for allocating memory for OpenGL textures. This change adds the buffer
size property in SurfaceInterface that can be used for allocating memory
for textures as well as monitoring buffer size changes.

I must say that the introduction of the buffer size property is a crude
hack because BufferInterface just needs to provide an OpenGL texture for
each plane. The main blocker for that is that it would involve moving
the backend, the compositor, and the wayland bits in the same place, for
example kwayland-server or ultimately kwin.