display: fix preview ghosting when rearranging monitorsHEADmaster

Dragging monitors in the display preferences left ghost images of the
previous position on systems without a compositor.

This patch removes creating an entire drawing context on the parent
window just for testing when edges touch and instead use a single-pixel
cairo surface. That way we avoid interfering with pending redraws. Then
we replace the manual canvas-to-window coordinate translation for
invalidation with a simple redraw call, which should handle windowless
widget positioning correctly.

Fixes #808

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