Some texture-related fixes

Bugs fixed in this branch:
* upper/lower/middle textures with non-existent texture ids being capable of crashing the game. For instance, RVZ1 has colormap codes on non-colormap linedefs, which causes them to wind up with invalid texture ids because of how the game tries to interpret lower/upper textures with "#" followed by characters on normal linedefs. Fortunately these "textures" are normally not visible anyway (since they're all in control sectors) unless they are swapped with in-level textures by some crazy Lua script of some sort...
* animated single-patch textures with holes displaying garbage on first viewing (see this thread: https://mb.srb2.org/showthread.php?t=42195)
* the heights of the lighting (shadows or colormapping) from water/translucent/shadowcasting/etc FOFs become messed up when displayed on repeated midtextures.

See merge request !144

Flat changing hotfix

This should fix how Lua scripts that change flats in levels, particularly to flats that don't already exist in the level, cause SRB2 to crash for anyone who joins a netgame afterwards. (I'm not sure if it's a consistent thing or not though)

Probably needs a good test to verify the issue is completely fixed before being merged in. Maybe there are other minor things I didn't account for in this fix? I don't know!

See merge request !141

Remove secplanes

This is just removing SSNTails's old port of ZDoom's secplane code and math, from back when he attempted slopes himself. The slopes we've had since 2.1.15 however do not need these, so we can pretty much toss the code for them out now (nothing uses them anyway).

See merge request !149

Netcode shit

Remove all traces of SSN's old port of ZDoom's secplanes for slopes, since our slopes do not need them

FixedRounding hotfix

Fix the FixedFloor, FixedTrunc, FixedCeil, and FixedRound functions.

See merge request !142

Savegame hotfix

This fixes the first mobj thinker added/read from $$$.sav not being relinked to other objects as their target/tracer/etc properly. In particular, this fixes Brak's electric shield disappearing for anyone joining a netgame playing ERCZ (as Brak happens to be the first object added/read in this case, and Brak is the shield's target).

See merge request !146

-Fixed broken net commands, thus fixing (or at least greatly improving) chat/commands/joins/leaves and possibly other annoying bugs
-Updated packet name list so the debug file no longer shows garbage packet names
-Replaced byte values with actual net command names in the debug file. Only the first net command in a packet will be shown though
-Added a MOBJCONSISTANCY define that makes the game takes all revelant mobjs to be counted in the synch seed
-Added a PACKETDROP define that adds two console commands "drop" and "droprate" to simulate bad internet by dropping packets
-Added/changed comments here in there in the netcode
-Fixed a minor error that would ignore one of the urgent ack slots
-Added a space between the map name and "zone" for the messages shown in a joiner's console

Start mobjnums at 1 instead of 0, so that the first found mobj can be relinked as a target etc to other mobjs properly

This fixes Brak's electric barrier disappearing for joiners to ERZC. There seems to be some issues with the lava falls there too I've found, but the electric barrier actually stays around now at least

(Fixes LSHIFT typing D repeatedly into chat)

Hook optimisation

Zlib pkgconfig

Linux build to include libgme by default

See merge request !116

Created R_CheckTextureCache to make sure midtexture/FOF walls cache their textures before choosing colfunc_2s, for software mode

Created R_GetTextureNum to make sure top/bottom/midtexture texture ids are always valid in rendering code for both software and OpenGL (and also for the Solid Midtexture effect physics code)

Generic mobj hooks are now run before mobjtype-specific mobj hooks, and player/linedef executor hooks now have their own lists

readme.txt duplicated, also incorrect

readme.txt should be README.md

No switching skins mid-credits/custom cutscene in SP

Does what it says on the tin.

See merge request !129

Smoother ropes and zoom tubes

Makes rope hangs and zoom tubes suck less. Specifically, they handle corners (and vertical height changes, ala sloped rope hangs) a LOT nicer. Ported from internal.

See merge request !139

As LJSonic has pointed out, there's no need for a for loop in either case; just use sector->floorpic/ceilingpic as a levelflats index directly

(Besides, if that was to stop any out-of-bounds indexes being used, that's hardly the way to do it anyway)

Also created P_CheckLevelFlat to just return the flat # from a name, since that's all P_NetArchiveWorld really needed from P_AddLevelFlat anyway

SDL2 i_video.c code cleanup

Made some code look a bit neater, removed the need for some SDL2STUBs Fury placed, vid_info console command actually kind of has a use again, etc

See merge request !138

Fix ceiling springs thrusting themselves down in midair on touch

What the title says. I blame MFE_ONGROUND for being stupid, but for now I made it behave a bit better with flipped items at least.

See merge request !140

This fixes ceiling springs apparently flying down with you in various scenarios

Revert "(Messiness warning) attempt to start using SDL_SetRelativeMouseMode instead of the old hacks of making the mouse do movement without leaving the window and be hidden"

This reverts commit 347b5318.

(Too experimental, may end up adding new bugs; let's just keep it to clean up for now)

(Messiness warning) attempt to start using SDL_SetRelativeMouseMode instead of the old hacks of making the mouse do movement without leaving the window and be hidden

Seems to work so far though

This way it's easier by eye to see from the list which SDL scancode maps to which SRB2 key code