Needs to be tested in gravity flip
Also flips the normal on linedef based slopes so that it's facing the correct direction
Also makes it so slopelaunch doesn't always remove the standingslope (going down some slopes would cause it to be constantly unset and reset)
This should hopefully alleviate the issue where textures overwrite the screen, or the screen overwrites the texture for now.
THIS IS NOT A FULL SOLUTION TO THE PROBLEM, but I highly doubt we'll ever be loading 4 billion textures at once, ever.
Other notes:
* on second thought I'll keep the hw_clip functions' gld prefixes rather than HWR, not like it matters either way
* despite the extra lag it does fix the issues with translucent walls and such when displayed at different vertical angles, such as with the GFZ1 waterfall
Other notes:
* Renamed all new functions to have HWR_ prefix instead of gld_, for consistency
* HWR_FrustrumSetup and HWR_SphereInFrustum are disabled and require HAVE_SPHEREFRUSTRUM. This is because 1) SRB2CB did not need the code, so presumably neither will we, and 2) there are some OpenGL API functions used there that due to our way of using OpenGL we don't use outside of r_opengl.c, which makes dealing with HWR_FrustrumSetup complicated in theory
* The new clipping functions are not added to OpenGL's "main" rendering code itself just yet, they're just available to use now once hw_clip.h is included
(Huge change: NO_MIDI is now a define, simply because that makes it easier for me to resolve merge conflicts if I can see the original code and the new code. Also means that people who REALLY miss MIDI can compile & fix it themselves :P)
* gr_md2 on the menu.
* Disable interpolerated model option completely for now.
Also, tweaked the GL menus a bit to suck less, since I was ferreting around in that area.
this fixes a crash in (old) GFZ2 at the ramp as a result of creating pv1/pv2. This probably means before pv1/pv2 there could have been some silly typecasting from vertex_t to polyvertex_t to get fixed vertex coords and such...
I also optimised those two functions while I was there (why keep a "floating" variable when setting it to false guarantees the functions return false?)
* 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.
