Card corners are not clipped when dragging

[BartMassey]

On dragging cards, some background color fills out the card corners. Not very noticeable except on xmille, where the corners are more rounded. Could fix with clipping or alpha, looks like.

[keith-packard]

alpha not so much as that would require compositing instead of simple window clipping. Shaping could happen, but is a bit hard as the card outlines are only an artifact of rendering and not really known independently.