Fix stale ghost content after window move/resize/close on real hardware

The udev backend's damage tracking only forced a full repaint (ages =
[0, 0]) on a workspace switch or a VT-switch resume. An ordinary
move/resize/open/close/restack within the same workspace relied
entirely on OutputDamageTracker's own per-element diffing to compute
correct damage for the region a window vacated - which doesn't always
hold: maximizing a window over a second one, then un-maximizing, left
a persistent ghost of the second window's old titlebar/status text
sitting in the vacated corner, unchanged across multiple otherwise-
idle frames.

render_udev_frame now also hashes every visible window's id and rect
each frame and forces the same full-repaint reset whenever that
signature changes - the same "defensive, not a fix for a proven bug
in the diffing itself" reasoning the existing workspace-switch reset
already uses, just triggered by a second, complementary condition.

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