ForceSkyClear as a map compatibility option

[Rachael]

Unfortunately it seems like I am stumbling upon maps like these more and more. Setting "gl_noskyclear 0" fixes the black boxes (which are HOMs) appearing at the top of the pillars.

This would fix the hallway near the end of the Sapphire map, as well, and quite likely a numerous few Stronghold maps.

Maps like these take advantage of rendering hacks that GZDoom would normally, by default, cover up.

I'm willing to try to do this one myself, I just need to know if such a feature would be a good idea or not. My idea is to make it a bcompat option, and not expose it to mapinfo or CVARs.

[Graf Zahl]

gl_no_skyclear should be 0 by default anyway. Setting this to 1 will have performance degrading effects because it requires a complete portal to render the sky.
In any case, I'd rather like to know why this doesn't work right, it looks like a problem with how the portal stencil is set up.