Do not alllow error codepaths of UPnP to leave the mutex locked

Closes #1200

See merge request !2349

Issue #1150 - Fix it the right way!

See merge request !2350

This reverts merge request !2334

Merge remote-tracking branch 'origin/1200-next-cannot-load-game-when-using-srb2-launcher' into 1200-next-cannot-load-game-when-using-srb2-launcher

Fix tint equation shader

See merge request !2348

Print UPnP status to log file/external console only

See merge request !2347

Delete outdated docs

See merge request !2346

Add UDMF specification document

See merge request !2345

Cutscene timing fixed

See merge request !2344

Fix FreeBSD memory reporting if built with cmake

See merge request !2314

I don't think It's not really important enough that it's worth always notifying the user about it.

OpenGL palette rendering and related things

See merge request STJr/SRB2!1516

Fix translucent pixels being visible on empty pixels

See merge request !2335

Issue1150

See merge request !2334

Fix #1193

Closes #1193

See merge request !2330

This was nothing more crazy than a buffer wraparound.

See merge request !2327

Update objectplace to ignore float argument and handle ZABSOLUTE

Closes #1111

See merge request !2338

Fix UPnP port mapping not working

See merge request !2333

dc_texheight must be (INT64) to prevent overflow and rendering softlock

See merge request !2323

Rename gr_allowclientshaders to gr_allowcustomshaders

See merge request !2319

Fix MOTD not being sent to players on first join

Closes #1194

See merge request !2317

Make the Cheats Command Reflect UsedCheats

See merge request !2160

Fix faulty references when spawning mobjs

See merge request !2126

HWR_ProcessPolygon()'s shader parameter was renamed to shader_target

cv_fovchange was moved from hw_main.c to r_main.c
cv_glallowshaders was moved from hw_main.c to d_netcmd.c

add back pic_t