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 STJr/SRB2!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

- DEBUGMODE
- MOBJCONSISTANCY
- PACKETDROP
- ZDEBUG
- PROFILEMODE (likely moot, dunno how profiling with cmake works
though)

cmake: Port warning configuration from make

See merge request !1844

Adds SRB2_CONFIG_ERRORMODE to replace ERRORMODE too.

Add_Scroller: Don't call IsSector3DBlock if affectee is a side

Closes #898

See merge request !1852

Fix typo in rising FOF binary-to-UDMF conversion

See merge request !1853

cmake: Remove asset hashing, CMAKE_ASSETS_DIR

See merge request !1847