implement Widget opacity #8
Description
Widget opacity is given as a float value from 0.0 to 1.0 with 0.0 meaning completely transparent and 1.0 meaning, completely opaque. All widgets with a value between these two extremes must register at the user interface to get a pre-render stage. In the pre-render stage, the widgets are drawn to an offscreen texture and those are drawn in the normal UI drawing stage with the opacity applied. This allows widgets to be differently opaque including their contents. Because pre-rendered widgets could contain other widgets that need pre-rendering (another opacity stage or even view_displays) the pre-rendered views have to be organized in a dependency hierarchy and rendered bottom-up.