Multiplayer Bot Features (aka AddPlayer)

See merge request STJr/SRB2!1383

Give Coins drop shadows

See merge request STJr/SRB2!1591

Previously took only the filename, so the directory
component was stripped. This broke dependencies for
basically every file.

Fix #492

Closes #492

See merge request STJr/SRB2!1431

Fix console aliases bypassing CV_NOLUA

Closes #619

See merge request STJr/SRB2!1573

Revert netvars to user value after demo finishes playback

See merge request STJr/SRB2!1578

Fix for #551

See merge request !1590

P_ZMovement: add a P_MobjWasRemoved check after P_CheckPosition, so we can bail out if the mobj was removed (by Lua most likely)

Actually tell you if the server is running an incompatible version

See merge request STJr/SRB2!1509

Lua userdata invalidate fix

See merge request !1584

Addplayer 2021 7 15

See merge request !1585

This was done by storing flag-for-removal status as a boolean inside the player struct.
Bot players are instead removed at the start of G_Ticker, rather than being removed immediately by G_RemovePlayer.

Bugfix - Fix sporadically occurring incorrect userdata types in Lua, caused by previously loaded userdata which didn't get invalidated in previous sessions.

Invalidate userdata for line and mapthing args.
Invalidate userdata for slopes and their normal, origin and direction vectors.

Use floating-point math for polyobject planes as well.

Closes #581

See merge request !1533

(Demos do not call CL_Reset BTW.)

And quote variable name.

Take advantage of new Makefile in appveyor.yml

See merge request !1561

Also changed -Wno-error=implicit-fallthrough to
-Wno-implicit-fallthrough. For some reason Appveyor's
version of GCC is triggering these warnings despite the
comments, so just shut it up.