Technically the Surface itself does not have a size, it's the
ShellSurface or the size of the FullScreenShell's Output. But it
simplifies a lot if we keep track of the size in the Surface as that
way we can hide the fact which kind of Shell is used.
The user of the Surface must connect either the FullscreenShell's
Output or the ShellSurface to set the size on the Surface.
A Surface class is split out which holds a wl_surface and supports
attaching a buffer, setting the damage and emitting a signal when the
frame callback got called.
It doesn't come with a unit test yet as it first needs the ShmPool
and Buffer properly split out to easily set it up.