layout: Resize the active surface when doing the split

This adds an intermediate geometry variable which is used to save and
restore to the original available geometry when destroying the split
surface. This takes care to inform the client that a new resize is
needed when the split surface is destroyed or when the split surface
is committed.

The width and height of the split surface is derived from the available
geometry area size (the available size of the background surface) but
this can further adapted to be based on the split window size (that is,
it could be specified by the client). This assumption is there to
simplify the implementation.

Bug-AGL: SPEC-3334

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ia484a922a7cbd314db2c878f81cb548458d1c1cd