I worked around the issue for the particle editor, but now it turned out
it can also somehow happen when switching from the Radiant to the Engine
(with F2), so I implemented the "proper" fix of restoring
glConfig.vidWidth/Height, that are overwritten in
RenderSystemLocal::BeginFrame(), in RenderSystemLocal::EndFrame().
I think I have a fix for this, though I couldn't always (but sometimes) reproduce this issue - it didn't always happen when switching to the engine with F2.. but OTOH I had an even worse bug (before I also fixed that): The game was frozen when starting it from the editor. Menu and Console were working, but I couldn't move ingame and neither did anything else in the level move..