PoC: The renderer should do alpha- blend to the background color

It is possible to set the global background color to the StateManager.

Currently, it does not work as intended, because rendering rewrites the background with the alpha when rendering SafePicture or SafeText. Instead, it should do the alpha blending with the background color.

In addition, the background clearing should be moved from the StateManager and to the application responsibility.

Within that approach, it would be possible to make sure that text is always readable despite the non-safe UI on the background.