Fix G_GhostTicker crashing trying to read netvars as ghost data

See merge request STJr/SRB2!1203

NPO2 span function optimization

See merge request STJr/SRB2!1201

Kick clients that fail to catch up with server gamelogic

See merge request STJr/SRB2!1208

Also fixed possible issue with incorrectly reading the demo pointer due to the wrong type being passed to sizeof in G_ConsGhostTic

Model fallback

See merge request STJr/SRB2!1167

Bustable Wall and Floor flags.

See merge request STJr/SRB2!1163

Performance stats

See merge request STJr/SRB2!1186

R_FindPlane optimization

See merge request STJr/SRB2!1199

Apply step up/down to opposite plane too

See merge request STJr/SRB2!1192

Lua polyobjects

Closes #19

See merge request STJr/SRB2!1140

Make record attack's quick retry more consistent.

Closes #80

See merge request STJr/SRB2!1193

Fix OpenGL GIF recording crashing with gif_localcolortable disabled

See merge request STJr/SRB2!1196

Fix gzip failing to compress SRB2's objdump on non-Windows.

See merge request STJr/SRB2!1191

Software 3D floor rendering optimization (second attempt)

See merge request STJr/SRB2!1184

Allow JumpSpin abilities to be used with non-ability shields

Closes #277

See merge request STJr/SRB2!1146

Save netvars

See merge request STJr/SRB2!1179

Give the addfile command variable argument support.

See merge request STJr/SRB2!1176

Shader code cleanup

See merge request STJr/SRB2!1103

I am going to die :D (closes #337)

Closes #337

See merge request STJr/SRB2!1183

Fix clients being able to skip credits AGAIN (closes #339)

Closes #339

See merge request STJr/SRB2!1182

A special stage is a special stage, even outside of coop.

Closes #211

See merge request STJr/SRB2!1180

Don't count demo playback as visiting a level.

Closes #327 and #340

See merge request STJr/SRB2!1177

Add `-splitscreen` commandline parameter

See merge request STJr/SRB2!1175

Add P_CeilingzAtPos function

See merge request STJr/SRB2!1164