request: restore r_mirroralpha GLquake functionality.

[BeholdersEye]

As John Carmack wrote in the GLquake release notes:

novelty features

These are some rendering tricks that were easy to do in glquake. They aren't
very robust, but they are pretty cool to look at.

r_shadows 1
This causes every object to cast a shadow.

r_wateralpha 0.7
This sets the opacity of water textures, so you can see through it in properly
processed maps. 0.3 is very faint, almost like fog. 1 is completely solid
(the default). Unfortunately, the standard quake maps don't contain any
visibility information for seeing past water surfaces, so you can't just play
quake with this turned on. If you just want to see what it looks like, you
can set "r_novis 1", but that will make things go very slow. When I get a

chance, I will probably release some maps that have been processed properly
for this.

r_mirroralpha 0.3
This changes one particular texture (the stained glass texture in the EASY
start hall) into a mirror. The value is the opacity of the mirror surface.

He is right. It is pretty cool to look at.

r_mirroralpha.mp4

And having the mirror effect available on that texture would allow mappers to access it by replacing the texture.

If it is "easy to do in glquake" I would like to see the feature restored in Quakespasm. It seems to have been killed because FitzQuake's developer thought it was a bad implementation but some might still prefer the bad implementation to none.