The button bevel was rendered upside down, which happens due to some
weird double y-axis inversion, which goes away when passing the QWidget
pointer to the style. When using the Qt backend, we can do that.
Some styles recompute the size based on the contents given in the QStyleOptionButton,
Some styles need to be given the icon in their content size. And this was not done properly
Set QStyle::State_Active conditional to whether the window is active,
which maps to Qt's active window concept and winit's has-focus (until we
render popups differently).
