Mingw32 support

See merge request STJr/UltimateZoneBuilder!15

Fix symbol lookup error caused by GL_GetProcAddress name being mangled by g++

See merge request STJr/UltimateZoneBuilder!16

3D Floor Mode: fixed a problem where the previous state of linedef arguments were not preserved for undo snapshots. Fixes #967

Script Editor: fixed a crash when the find/replace dialog was opened and the previously selected script tabs do not exist anymore. Fixes #993

Sound Propagation Mode: fixed a problem where removed sector geometry was still shown when undoing/redoing. Fixes #992

Fixed a problem where portal render style could not be set for the Eternity Engine game configuration

Visual Mode: fixed a problem where texture skewing didn't work correctly for textures scaled through sidedef properties. Fixes #985

fixup on removing COMMONENV

FORCEINLINE is not a _WIN32 thing
It is a compiler attribute
use always_inline for GCC
__forceinline for MSVC
and plain inline for others

also cast glGetError via void *

libsimde-dev is not packaged for Debian buster

clear out warnings for _WIN32 codepath

Debian buster have the *-mingw32-g++-posix in the package g++-mingw-w64-i686 and g++-mingw-w64-x86-64

Build Win32 BuilderNative.dll with Mingw-w64

Update .gitlab-ci.yml file

See merge request STJr/UltimateZoneBuilder!14

Script Editor: pressing enter in the search box now performs the search. Pressing enter in the search box of the replace tab performs a search. Pressing enter in the replace box of the replace tab now performs a replacement. Pressing the Escape key now closes the dialog. Fixes #972

Visual Mode: added support for GZDoom style texture skewing

Eternity Engine Game Configuration: fixed an issue where UDMF sector properties were not labeled as unmanaged

Sound Propagation Mode: fixed a crash when things were outside the map's geometry bounding box. Fixes #975

Edit Linedef dialog (UDMF): fixed a problem where the back sidedef's controls were not disabled correctly if their UDMF fields were not set in the game configuration