winex11: Move GL/VK offscreen if the clipping region is NULLREGION.
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 786d9d16)
Fixes: 786d9d16 Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=57503
Edited by Rémi Bernon