Ramp wall weirdness in OpenGL

These weird sloped wall issues pop up in various places when using the OpenGL renderer.

mentioned in issue #195 (closed)

added Bug OpenGL renderer labels

mentioned in issue #234 (closed)

mentioned in issue #245 (closed)

mentioned in issue #405

Light blocks seem to be a common cause of this issue. Maybe not the only cause, but here are some examples from MB bug reports where light blocks are likely the culprit:

And a couple of gifs:

https://mb.srb2.org/attachments/srb20408-gif.20782/

https://mb.srb2.org/attachments/srb20405-gif.20779/

https://mb.srb2.org/attachments/srb20407-gif.20781/

Probably caused by HWR_SplitWall (splits walls from light blocks) having issues with splits that don't form quadrilaterals.

Possibly related situation in CEZ1:

For comparison, in Software:

mentioned in merge request !1781 (merged)

mentioned in commit 1f97ee3f

closed with merge request !1781 (merged)