Add Dockerfile for Windows cross compilation

[edubart]

Steps to build

1. Build the windows toolchain image and open a shell for it:

make build TARGET=windows
make run TARGET=windows

2. Inside the shell compile 2dcg-src

cd host/2dcg-src
make \
    PKG=x86_64-w64-mingw32-pkg-config \
    CC=x86_64-w64-mingw32-gcc \
    CXX=x86_64-w64-mingw32-g++ \
    LUA_INC="-DRVG_LUAPP" \
    LUA_LIB=-llua \
    B64_INC= \
    B64_LIB=-lb64 \
    SOLDFLAGS="-shared -fPIC" \
    QT6_MOC=/usr/lib/qt6/moc \
    vg_build_qt5=no \
    vg_build_hyphenate=no \
    all -j$(nproc)

3. Create a windows distribution containing all 2dcg dependencies:

mkdir -p windows
cp /usr/x86_64-w64-mingw32/bin/lua.exe windows/
cp /usr/x86_64-w64-mingw32/bin/*.dll windows/
cp -R /usr/x86_64-w64-mingw32/lib/qt6/plugins/* windows/
cp -R /usr/x86_64-w64-mingw32/lib/qt6/qml/* windows/

4. Inside a windows machine, clone 2dcg-src (so you have process.lua) and copy windows to it, then test it:

cd 2dcg-src
export LUA_PATH_5_3="?.lua;;" LUA_CPATH_5_3="?.so;;" WINEDEBUG=-all
wine windows/lua.exe process.lua svg 2dcg-rvgs/triangle1.rvg
wine windows/lua.exe rvgv.lua 2dcg-rvgs/triangle1.rvg