NULLREGION means that the entire window should be clipped out, and its
children displayed instead. The change broke some offscreening decisions
when both parent and children have a GL drawable created but drawing
actually happens on the child window.
(Found while trying to reproduce otherwise unrelated regression in
https://bugs.winehq.org/show_bug.cgi?id=52738, the game there uses ddraw
and doesn't display anything after 786d9d1685ac220081b10cc779d4d331ddd2fc52)
Fixes: 786d9d1685ac220081b10cc779d4d331ddd2fc52
Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=57503
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v2: winex11: Move GL/VK offscreen if the clipping region is NULLREGION.
winex11: Use DCX_USESTYLE when checking DC clipping regions.
https://gitlab.winehq.org/wine/wine/-/merge_requests/7014