Add opacity_group hack

Add opacity_group hack

This adds a way to get the gtk_widget_set_opacity liike behaviour
of retargeting GdkWindows and exposing every child in ::draw, without
actually having an alpha. This is needed if you're doing more complex things
such as cross fading of widgets.

We do this as a hack by using opacity values that round to 255 yet not
really 1.0 in order to avoid having some magical API call for this
mainly internal call.

https://bugzilla.gnome.org/show_bug.cgi?id=687842