Mirrors don't work with dynamic glow enabled

[xycaleth]

As title describes.

The issue is this if block in tr_backend.cpp: https://github.com/JACoders/OpenJK/blob/master/codemp/rd-vanilla/tr_backend.cpp#L483

Need to determine why the if block is there - it mentions there being a bug when rendering multiple scenes, which suggests this is more of a hacky fix than something that's necessary - and if it's possible to fix the bug it mentions, so that this can be removed.

[SzicoVII]

This is definitely true, always had an issue with this...

[ensiform]

Apparently atlantica mirrors work fine @xycaleth @SzicoVII but blueice nightfall ones do not. Shader issue it seems perhaps? I know that one uses a map $lightmap stage and one does not.

[ensiform]

@xycaleth fog related actually isn't it?