Draft: qt: blend static shadow with the background ourselves rather than graphics backend

When we blend ourselves, QSG batch renderer's job becomes easier because the shadows become opaque as long as the background color is opaque, which in turn disables blending (https://github.com/qt/qtdeclarative/blob/2d9c5c46d34b90510f3af31f2dd01ae15226c6e0/src/quick/scenegraph/util/qsgtexturematerial.cpp#L169).

This is currently a prototype and has issues with the z-order. We can only use this when the shadow is an immediate overlay on the background.