Major Opium update

Continuing where @ferdinand.list (aka cityy) left off, with their kind permission. Quote from cityy:

When I was redoing my level design project archive, I noticed that the screenshots I have of opium do not match with the version of the map that was added officially to the game. It appears that in 2013 I was doing additional detail/texturing work and some tweaks to the gameplay that I apparently forgot to release as opium v5. I recovered this version from an old hard drive and tested that it compiles and runs fine in the current version of xonotic. Notable changes compared to v4 include the Large Armor platform being attached to the upper corridor and a second water route being added between center tunnel and the large teleporter. There is also a dark version of the heaven skybox in this. I know I should have possibly found this a few years earlier as there was additional work done on v4, sorry about that! I don't know if this version is worth retouching again to be added to the game but I'm putting it out there anyhow just in case.

Cityy's initial branch was cityy/opium_v5, I forked it and have been cleaning up the map & shaders a bit.

Targeting Xonotic 0.9