agl-shell-desktop: Add the ability to pass width and height as

agl-shell-desktop: Add the ability to pass width and height as
area of the bounding box

The width and height will represent a rectangle which can be used to
define the maxium area to have the surface in. Any part of the surface
area that exceeds the area delimited by this bounding box will be
clipped away. The initial position of the bounding box is specified
using the bx and by values.

Bug-AGL: SPEC-3419

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ie24f469662dd686c49f658c415ccd2664d9a9b5b