Sliders: render left-hand-side darker

[mickleness]

Many sliders these paint the left hand side a darker color than the right hand side.

The ShowcaseExampleDemo#ShowcaseSlider may already do this for Windows.

We could generally do the same thing on Mac using the VectorGraphics2D to intercept and hijack the calls to paint the track.

(Also, maybe a separate ticket: the thumb position of sliders on Mac lines up with the extreme left/right of the component. This makes UI lines cleaner (there's not a lot of extra padding on the left/right). This may be a harder thing to change, though...)