MetaWaylandSurface: cache the surface type until the first commit

Once the sizing is properly wired up, we need to make sure that
the size at the initial map is correct, and not always 0, 0 because
the buffer is not yet converted into a CoglTexture by MetaShapedTexture,
otherwise we end up sending out configure events at 1 x 1.
To do so, we cache the surface type in the initial state until the
first commit.

https://bugzilla.gnome.org/show_bug.cgi?id=707401