Implement rendering of feather selections

Lightbox and highlight rendering now support feathered selections, which
means all feathering additions in the future will work without extra
coding!
Implement full antialiasing for elliptical selections

GDI+ makes this fairly trivial, though there are some quirks like
converting RGB() data to the RGBQUAD GDI+ requires.

Rounded rectangles are next, though I have to manually draw them via
paths as GDI+ has no inherent DrawRoundRect function.  <sigh>