More slope fixes (aka sorry guys I made another quick SUGOI fix)

Another slopes fix branch!

This branch currently includes a fix for:
* Knuckles gliding into a slope while in 2D mode causes him to try to "climb" on air above them, if the original non-sloped height is higher. Unfortunately he still tries to grab onto places that he can't really climb on, but at least now it's on the slope itself and not in mid-air lol. This issue is encountered in SUGOI's Retro Hill Zone, if you want to check for yourselves.
* Bustable FOF-busting code for both players AND pushables not accounting for slopes
* (Unrelated to slopes): Fix FF_SHATTERBOTTOM FOFs acting like THZ goop when stood on; I added this fix as a bonus because I encountered it in a test map of mine for the bustables fix

(Other fixes may or may not appear here in the near future, I haven't decided yet. Don't wait on me to get in any further fixes before merging, that said)

See merge request !110

Last-minute NiGHTS exiting bugfix

One single-line fix concerning the end of NiGHTS maps. Extremely small, literally zero side effects here, the only reason I'm not committing directly to master (aside from ettiquette) is that I don't have the ability to do so.

* The NiGHTS drone had a single tic of visibility when you hit the goal, which is evident stepping frame by frame through http://gfycat.com/ComplicatedComposedAoudad (the contents of which may or may not make it into 2.2). This is no longer the case.

See merge request !100

DIsable NetPlay on DirectDraw builds

Also disable UPnP support on NONET builds

See merge request !102

Readme rewrite

Our readme.txt is outdated as hell, so I took it upon myself to rewrite it in Markdown. For the sake of simplicity, it links to the wiki for compile instructions. Feel free to commit to the readmerewrite branch if anything should be done differently.

See merge request !111

Optimisation of P_GetMobjGravity (relevant for slopes)

Cleaned up some performance issues in Skytop Zone (2.1.16: THE SUGOI UPDATE) arising from some inefficiencies with P_GetMobjGravity. FPS drops only occasionally to 34 now, which is a big improvement when I was frequently getting 27 in Salt's 15andahalf.exe.

(I also tested in a build of 2.1.16's first release candidate, and was getting drops to 16! So SOMETHING improved since then, but this was the major issue.)

See merge request !109

Record Attack hotfix

Limit damage from http://mb.srb2.org/showthread.php?t=41847, basically.

Also fixed the message popup background because why not

See merge request !107

Fix Knuckles in 2D mode attempting to latch onto air when gliding into a slope. Now he just attempts to latch onto the slope directly (which I'll fix another time, if I can figure out how)

(fix for SUGOI's Retro Hill Zone)

Cutscene switch fix

Yet another fix for SUGOI, lol: apparently going from a final level's post-cutscene straight to a custom credits cutscene gets the latter stuck forever in the first scene (because the game accidentally stops the cutscene responder from realising the custom credits cutscene is running). This branch of course makes a fix for that.

See merge request !108

Wall collision fixes

Fixes included in this branch:
* A fix for a specific crash encountered in a SUGOI map (sound familiar?) caused by bouncing off a wall adjacent to a slope
* A fix for solid midtextures not accounting for Effect 3/Peg Midtexture properly (https://mb.srb2.org/showthread.php?t=41462)
* A fix for solid midtextures not accounting for texture y offsets properly for non-Lower Unpegged/Peg Midtextured midtextures
* A fix for solid midtextures not accounting for "infinite" repeats (Repeat Midtexture + no repeat count set)
* ~~A fix for Effect 4 on Polyobject First Line making that particular linedef's midtexture solid in addition to making planes visible - this is not wanted if you want a polyobject with both visible planes and full intangibility.~~ Apparently they never did this anyway, don't mind me \o/

See merge request !104

Cleaned up some performance issues in Skytop Zone (2.1.16: THE SUGOI UPDATE) arising from some inefficiencies with P_GetMobjGravity. FPS drops only occasionally to 34 now, which is a big improvement when I was frequently getting 27 in Salt's 15andahalf.exe.

Climbing shit 2 electric boogaloo: an adventure in how much toast hates how the ogl renderer was implemented

Fixed Knuckles being able to climb in space in OpenGL.

Using the last-touched line instead of R_IsPointInSubsector because FLOAT_TO_FIXED is bad in P_ functions

To understand the original problem: look at AjustSegs(void) in hw_bsp.c. It reallocates the vetex_t pointers for lines as POLYVERTEX_T pointers, and of COURSE things are gonna get wacky when you're casting pointers.

This merge request is into master because it's a bug that's IN master and must never reach the public because of how bad it is

See merge request !101

Fix non-player objects having busted step-up/down on slopes

Why the fuck did I make this a player conditional in the first place holy shit

See merge request !103