CMake: Fix nasm Linux builds

When compiling under Linux with CMake and nasm/yasm enabled, the operation would fail during linking with undefined symbols related to the tmap files.
This commit adds support for passing flags to the assembler and passes ```-DLINUX``` in order to compile.

After this change, binaries are successfully compiled with either nasm or yasm on Linux systems.
Tested on Ubuntu.

See merge request !153

CMake: split inline ASM code from vid_copy.s code

See merge request !151

Update Code::Block project

See merge request !145

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

-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

(Fixes LSHIFT typing D repeatedly into chat)

Hook optimisation

Zlib pkgconfig

Linux build to include libgme by default

See merge request !116

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

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

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

(Apparently SDLK_LMETA/SDLK_RMETA don't exist anymore in SDL2 anyway?)

This means the console command vid_info also works properly too now (well, it does nothing in OpenGL mind)