Fix G_GhostTicker crashing trying to read netvars as ghost data

See merge request !1203

NPO2 span function optimization

See merge request !1201

Kick clients that fail to catch up with server gamelogic

See merge request !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 !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 !1199

Apply step up/down to opposite plane too

See merge request !1192

Lua polyobjects

Closes #19

See merge request !1140

Make record attack's quick retry more consistent.

Closes #80

See merge request !1193

Fix OpenGL GIF recording crashing with gif_localcolortable disabled

See merge request !1196

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

See merge request !1191

Software 3D floor rendering optimization (second attempt)

See merge request !1184

Allow JumpSpin abilities to be used with non-ability shields

Closes #277

See merge request !1146

Save netvars

See merge request STJr/SRB2!1179

Give the addfile command variable argument support.

See merge request !1176

Shader code cleanup

See merge request !1103

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

Closes #337

See merge request !1183

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

Closes #339

See merge request !1182

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

Closes #211

See merge request !1180

Don't count demo playback as visiting a level.

Closes #327 and #340

See merge request !1177

Add `-splitscreen` commandline parameter

See merge request !1175

Add P_CeilingzAtPos function

See merge request STJr/SRB2!1164