[SUGOI] Level select no longer assumes that games will always be linear

See merge request !1492

[SUGOI] Make stagefailed more useful, add linedef executor to toggle, and expose to Lua

Closes #361

See merge request !1463

[SUGOI] Remove skin->availability, add SECRET_SKIN

See merge request STJr/SRB2!1474

Fix slope plane rotation and alignment (resolves #508)

Closes #508

See merge request !1464

Fix some compiler warnings

See merge request !1504

Check against null tmpusher source before attempting to push a thing.

See merge request !1413

tRNS chunk fix

See merge request !1393

Optimize Picture_GetPatchPixel

See merge request !1342

Remove music slot compatibility

See merge request STJr/SRB2!1321

Scale minimum dashmode thok speed on actionspd, not normalspeed

See merge request STJr/SRB2!1311

Set the target of a spawned ghost to where it came from.

Closes #517

See merge request !1358

Make steam apply sprung flag

See merge request !1365

Don't apply pw_justlaunched if the player didn't actually launch off a slope

See merge request !1447

No fire shield palflash (resolves #512)

Closes #512

See merge request !1473

Change Raspberry's chat colormap to V_ROSYMAP

See merge request !1441

Add more actions for slope copying

See merge request !1406

Add linedef specials for multitagging in binary maps

See merge request !1485

The year is 2021

See merge request !1500

Some of these integers exceed the precision of float. In
that case the number is rounded. The rounding shouldn't
matter too much anyway, so just shut the compiler up.

Fix a series of buffer overflows in SOC (primarily Character section) loading

See merge request !1457

arccosine Lua exposure

See merge request STJr/SRB2!1455

[SUGOI] Allow setting sound captions in SOC

See merge request !1469

Monster cleaning up again

See merge request !1433

Load netgame error fixes

See merge request !1482

Polyobject over 3D floor fix for software renderer

See merge request !1488

2.2.9 prep

See merge request STJr/SRB2!1499