Cairo doesn't seem to deal correctly with an HDC produced
by BeginPaint() that has a clipping region. The problem affects
only Win64. Work around the problem by drawing to a separate
HDC and copying to/from the screen. (To see the problem before
this patch, draw the DrRacket window to the edge of the screen
and back, and observe tha the toolbar doesn't update correctly.)
This change could affect performance, but it should mostly
be limited to refresh when a window moves.
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