SButton and SBorder multiply color and opacity twice

Currently, the SButton and SBorder widgets multiply the color and opacity during OnPaint before calling the super (SCompoundWidget::OnPaint). This causes the operation to be done twice, since the colors are already being multipled by SCompoundWidget.

1. Create a Border or Button widget
2. Create a Text Block widget and child it to the border/button
3. Set the "Color and Opacity" field of the border/button to (0.5,0.5,0.5,1.0)
4. Note that the color of the text appears darker, and the color picker shows (0.25,0.25,0.25,1.0)