Integrate to read shadow parameters passed from decoration in KDecoration2::DecorationShadow

[paulmcauley]

Rather than setting the shadow and outline separately in ShapeCorners, ShapeCorners could be integrated with the KDecoration API and then read the shadow image, padding, and innerShadowRect that the decoration passes via KDecoration2::DecorationShadow.

This would be mean that ShapeCorners would support the exact shadow designs (many of them complex and non-symmetrical) in the plethora of existing window decorations already available, without having to tweak shadows and outlines separately.