VIGS: Support underlays and alpha blending

VIGS: Support underlays and alpha blending

z-pos of planes has new meaning, if
it's >= 0 - plane is an overlay, i.e. it's layered
above root surface. if z-pos is < 0 - plane is underlay,
i.e. it's layered beneath the root surface. This new
logic has no meaning without alpha blending, so it's
supported as well (for all types of planes)

Change-Id: I61a80b48a1ef6c486fc83d0190d0bd5c1b7d4472
Signed-off-by: Stanislav Vorobiov <s.vorobiov@samsung.com>