Reasoning:
* They're horribly broken (they completely ignore slopes).
* A similar final demo GL-only feature (coronas) has been completely disabled.
* Nobody's going to bugfix them before release because not even the devteam's resident GL users use them.
* They look ugly.
- Split MF2_DONTDRAW into MFD_DONTDRAWP[1-4], also replaces MFE_DRAWONLYFORP[1-4].
- Split MF2_SHADOW into MFD_FULLBRIGHT and MFD_TRANS80. I also added an entire spectrum of options for transparency & brightness overrides, since I've found myself wishing for stuff like that before.
- Tethering was updated for it's client-sided drawing to apply to splitscreens too.
- Removed cv_transparency.
The transparency overrides don't seem to work yet (obvious on things using MFD_SHADOW), just running out of time to look into it so I'm pushing what I have.
* Like Kart, remove cv_precipdensity.
* Like Kart, replace "Infinite" draw distance value with "None".
* Better thinker with more return optimisation.
* Better placement of thinking in rendering, to avoid ceiling-mounted sprite glitches.
The skybox rendering process has been replaced with portals instead. Those are generated after the first BSP tree pass by looking for existing sky visplanes at the time, and their windows are used to define new portals.
The skybox portals are still incomplete and cause visual glitches when masked elements are involved.
* Multiplied rain speed by 3, per Oni's request.
* Disable weather density - force to 1 if weather draw distance, otherwise zero
* Move the ceilingpic check into a more convenient part of the weather spawning loop.
* `drawdist_precip_cons_t` - replaces "Infinite" with "None".
* Disable the lowest normal draw distance (256), given... both kart and srb2 are basically unplayable like that.
* Disable cv_drawdist_nights entirely.
