[4.5pre devbuilds] Nvidia Quadro and gl_no_skyclear

[wildweasel]

Reporting this by proxy for somebody else:

Recent versions of GZDoom (stated to be within the last few days' worth of dev builds from DRD Team) were exhibiting bizarre framerate loss on Nvidia Quadro cards, eventually discovered to be happening any time the sky was in view. With some experimentation in the console, we discovered that gl_no_skyclear needs to be set to True in order for framerate to hold steady.

While the problem is solved for this person, I feel it worth exploring potential causes or fixes - or else a suggestion that, if GZDoom detects a Quadro, this cvar should default to True.

[Graf Zahl]

This sounds like a driver bug. gl_no_skyclear forces rendering the sky as a portal - normally, if there's only a single global sky it gets rendered into the empty framebuffer before the scene.
Forcing this cvar to true is not a solution, it'd only hide the real issue.