Add Windows cross-compilation via MinGW, and fix Docker build permission

issues on vboxsf-mounted checkouts

New `make dist-win`/`package-win` targets (folded into `dist`/`package`
alongside the renamed `dist-linux`/`package-linux`) cross-compile
systemshock.exe via MinGW, using prebuilt SDL2/SDL2_mixer/GLEW/
fluidsynth-lite baked into the build-image - no Windows machine or Wine
needed to build it, confirmed working and playable on a real Windows
machine. dist-win/ ships DLLs flat alongside the exe plus a new
res/run.bat launcher, packaged into a .zip the same way dist/ becomes a
.tar.gz.

Also fixes three build-image bugs hit while testing on a VirtualBox
vboxsf-mounted checkout: build-engine.sh/docker-entrypoint.sh losing
their execute bit (chmod +x on restrictive source perms), the container
user missing access to /workspace's supplementary vboxsf group, and
cp -a failing on symlink/hard-link creation (vboxsf doesn't support
either) - now falls back to dereferencing copies when detected. Also
adds Docker/zip/etc. prerequisites to the README for both the desktop
and Quest builds.

NOTICE.txt

@@ -16,3 +16,19 @@ The Android build additionally bundles:
 - The Khronos Group's OpenXR-SDK loader
   (https://github.com/KhronosGroup/OpenXR-SDK), prebuilt unmodified as
   lib/arm64-v8a/libopenxr_loader.so. It is Apache 2.0-licensed.
+
+The Windows build (cross-compiled via MinGW - see README's "4.2. Windows
+cross-build") additionally bundles:
+
+- GLEW (http://glew.sourceforge.net/), unmodified, as glew32.dll - used
+  for OpenGL extension loading, which Windows' own opengl32.dll doesn't
+  provide past OpenGL 1.1 (Linux instead gets this straight from Mesa's
+  headers, needing no separate loader library - see
+  engine/src/MacSrc/OpenGL.cc). It is licensed under a combination of the
+  Modified BSD License, the MIT License, and the Khronos License, all
+  permissive.
+- The MinGW-w64 runtime's winpthreads library, as libwinpthread-1.dll. It
+  is MIT-licensed. (GCC's own runtime, libgcc/libstdc++, is linked
+  statically into systemshock.exe instead of shipped as a DLL, under the
+  GCC Runtime Library Exception - this doesn't subject the rest of the
+  binary to the GPL.)