Redmond: borders aren't pixel perfect

[wandrien]

The "shadow out" and "shadow in" borders for frames and tabs (but not for buttons, scrollbars etc) have way too black shadows. This is because the rendering code for draw_box_gap() and draw_shadow_gap() wasn't ported to cairo and doesn't do the correct color blending. In fact, the redmond engine misses these two methods at all and the default Raleigh implementation is used.