The weston_surface structure is split into two structures:

 * The weston_surface structure storres everything required for a
   client-side or server-side surface.  This includes buffers; callbacks;
   backend private data; input, damage, and opaque regions; and a few other
   bookkeeping bits.

 * The weston_view structure represents an entity in the scenegraph and
   storres all of the geometry information.  This includes clip region,
   alpha, position, and the transformation list as well as all of the
   temporary information derived from the geometry state.  Because a view,
   and not a surface, is a scenegraph element, the view is what is placed
   in layers and planes.

There are a few things worth noting about the surface/view split:

 1. This is *not* a modification to the protocol.  It is, instead, a
    modification to Weston's internal scenegraph to allow a single surface
    to exist in multiple places at a time.  Clients are completely unaware
    of how many views to a particular surface exist.

 2. A view is considered a direct child of a surface and is destroyed when
    the surface is destroyed.  Because of this, the view.surface pointer is
    always valid and non-null.

 3. The compositor's surface_list is replaced with a view_list.  Due to
    subsurfaces, building the view list is a little more complicated than
    it used to be and involves building a tree of views on the fly whenever
    subsurfaces are used.  However, this means that backends can remain
    completely subsurface-agnostic.

 4. Surfaces and views both keep track of which outputs they are on.

 5. The weston_surface structure now has width and height fields.  These
    are populated when a new buffer is attached before surface.configure
    is called.  This is because there are many surface-based operations
    that really require the width and height and digging through the views
    didn't work well.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>