Cut inner walls of splat FOFs

Closes #891

See merge request !1829

Fix hidden 100% completion reward actually being obtained at 50% completion

See merge request !1842

Fix addon menu down arrow weirdness + allow looping from top to bottom and vise versa

Closes #829

See merge request !1848

Merge master into next

See merge request !1861

Ignore CMake's build folders

Fix NONET build

Plug up some holes with lib_cvRegisterVar

Closes #441

See merge request !1823

HWR transparent FoF plane brightness

See merge request !1857

Uncapped

See merge request !1856

# Conflicts:
#	src/dummy/i_system.c
#	src/win32/win_sys.c

cmake: build zlib manually in internal build

See merge request STJr/SRB2!1856

cmake: Port misc options from Makefiles

See merge request !1855

Too common of a build pattern to ignore