[OGL] FlatSky rendering support

[JPL]

This is a nice effect and indicative of flexibility that could be used for various interesting visuals.

It'd be nice if WADs could provide their own shaders for skies, so you could do whatever you like there: PSX Doom style pixel fire sky, ZDoom-style flatsky, Quake1 style warping parallax sky, etc. It's always rendered behind the main scene so it's a little more clearly separable in the render process than, say, flats or things or wall textures.

No idea if that is any easier with the new shader changes going in, or how data-driven that stuff will ultimately be. I'm just thinking about what might be possible.

[ibm5155]

kinda like my situation, I modified the software shaders for making a unique shader style, but I cannot introduce that effect under my .wad/pk3 file, instead I'll need to release a modified gzdoom.pk3 file or just let alove people without the modified shader file
EDIT: oh there's (just saw the example of the "XXYYZ"? mod and I just did a copy and paste from what I changed)