Default: avoid use of opacity for overlay scrollbars

Default: avoid use of opacity for overlay scrollbars

If using the opacity CSS property the renderer cannot optimize these
handles without the use of offscreens due to the use of both a border
and rgb render node.

Instead, we can apply the alpha to the color values and get the same
effect in a way that the GL renderer can optimize without the use of
offscreen textures for a sizeable reduction in runtime overhead.