Fixing various problems with MF_SOLID collision

(I, toast, wrote the original merge request, but Alam made this merge request because I made a mistake. Twice. In a row.)

* Solid objects are now no longer intangible when your z is less than the other object's.
    * I originally thought the bug was limited to just solid objects on slopes (hence the branch name), and was wondering about undesired side effects - but looking at this gfy, this was a major bug that slopes made easier to expose.
    * Unfixed = http://gfycat.com/BareLimitedCavy
    * Fixed = http://gfycat.com/JubilantOffensiveGar
* You now properly lose your momentum when jumping at a solid object.
    * The way I fixed this was a hack, but it was originally a much worse hack which resulted in very bizzare physics issues, so...
    * Unfixed = http://gfycat.com/ShockingAbsoluteArthropods (also demonstrates the first bug)
    * Fixed = http://gfycat.com/EmbellishedCourageousGordonsetter

See merge request !119

Fixed players disappearing when spindashing whilst wearing multi-layer shields

* Before: http://i.imgur.com/gxUFElo.png
* After: http://i.imgur.com/NxsOg94.png

The problem was that MT_OVERLAY's default radius and height were never getting changed from 1*FRACUNIT, and that meant that when you spindashed, the game considered it completely below the surface of the flat you were standing on. Since you're not usually clipped on flats that don't belong to FOFs, we didn't notice this aspect of the issue sooner.

(SORRY FOR THE MULTIPLE MR'S AGAIN I TOTALLY SUCK AT REMEMBERING TO MAKE SURE IT GOES TO NEXT INSTEAD OF MASTER)

See merge request !124

Fix ANIMDEFS process order

v.drawScaled negative scales fix

Lua now errors if you try to use negative scales with v.drawScaled, rather than crashing as it does in 2.1.16 currently. Not much else to say here.

See merge request !126

Not in P_SetupLevel. That's just dumb.

ANIMATED and ANIMDEFS are now processed in reverse order,
and duplicate definitions in ANIMDEFS are dropped.

Fixing FF_REVERSEPLATFORM clipping fixes

Forgot the case where == 0.

Tested by @wolfy852 in Top Down, where the bugs this caused were first discovered.

See merge request !121

Even more slope-related fixes

What it says on the tin pretty much:

Currently included fixes:
* Pusher specials applied to FOFs did not properly account for slopes applied to the bottom plane; this probably was just a minor slipup that appeared on adding the support in the first place, I take it? (some waterfall in the last section of SUGOI's Fudge Canyon is the only place I can think of offhand where this bug is noticable)

See merge request !114

Treating " as whitespace in TEXTURES (and animdefs too i guess)

SLADE adds this character in its TEXTURES editor and makes SRB2 cry unless the lump is manually edited; this just treats it as whitespace so we don't have to think too hard about it.

Also, two I_Errors now refer to the correct lump name.

See merge request !122

Fixed that really annoying bug where you disappear when spindashing on top of a FOF whilst wearing a multi-layer shield.

(the problem was that MT_OVERLAY's default radius and height were never getting changed from 1*FRACUNIT, and that meant that when you spindashed, the game considered it completely below the surface of the flat you were standing on. Since you're not usually clipped on flats that don't belong to FOFs, we didn't notice this issue sooner.)

Fix demo recording in Linux

Fixed the problem with the reverseplatform_clipping branch that caused springs to fall through platforms, as tested by Wolfs in TD.

Prevent overflow when attempting to calculate the space between floor and ceiling. (I didn't notice any bugs as a result of the previous behvaiour, but you never know.)

Fixed the thing where if you thok into a solid object you spin really fast in the air without moving OR losing your momentum.

HOWEVER, since these changes to PIT_CheckThing do raise questions about whether there may be unintended side effects here. As a result, I may remake this for internal only if necessary.

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