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

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.

According to this answer--
https://stackoverflow.com/a/45952425
--Make will always prefer a unix shell, even on Windows,
if one can be found in Path. So we can't check PATH to
determine if it's a Windows shell... this is just too much
bother.

Use return value of KeyDown and KeyUp hooks

See merge request !1559

Some issues with the Makefile

See merge request !1553

If a variable is defined as in 'make CC=gcc-10', then that
definition overrides anything other definition in the
Makefile.

After a checkout from before revision, old directories
such as bin/Linux64 only remain if untracked files exist
within. This may be confusing to the user. They may even
use an outdated executable if it is one of those untracked
files.

Hooklib refactor

See merge request !1307

This is for MSYS2, which requires unix shell commands.

File function is not supported < Make 4.2.

# Conflicts:
#	src/lua_hook.h
#	src/lua_hooklib.c

Let Lua access key presses, mouse properties, and joystick properties

See merge request !1436

# Conflicts:
#   src/lua_script.c

Restore toggle for Robo-Hood to not jump away from nearby players.

See merge request STJr/SRB2!1548

Ignore a linedef tag of 0 when using linedef action 96.

See merge request !1543

With added textual contrast.

NOOBJDUMP=1 and NOUPX=1 have been removed. Make 'dump'
target to disassemble. UPX=upx to compress executable.

Setting UPX used to cause it to fail. That has been fixed.