Out of curiousity, does OpenGL mode render the back-sides of level stuff or not? If not, it might be interesting to find an area where there's a hole, then compile a version of the game that doesn't use back-face culling, and then check the same area in that version, to see if the hole is "simply" back-sided instead of front-sided. I'm not sure how easy that back-face cull disabling change would be in the code, but if it's easy enough, I think that it's worth testing that.