#618 GTK3V2MAIN: mswlib does not compile with modern MSVC — Win32 backend needs porting

[Allen D Byrne]

The mswlib Win32 backend in GTK3V2MAIN fails to compile against modern
MSVC (VS 2022, Windows SDK 10). Errors span the entire library:

1. undefined struct/union 'control' in mswbox.c, mswbutt.c, mswbitmap.c,
   mswchoic.c, and many others — the internal control struct definition
   is not visible. mswint.h likely needs to be added to the include path
   in app/wlib/mswlib/CMakeLists.txt.

2. Signature mismatches between mswlib implementations and wlib.h
   declarations (e.g. wButtonCreate redefined with different types) —
   the GTK3V2MAIN wlib.h public API evolved past what mswlib implements.
   All mismatched functions need to be audited and updated.

3. BC_HORZ undeclared identifier (mswchoic.c) — old Windows API constant
   removed from modern Windows SDK. Needs replacement or #define guard.

4. mswbitmap.c:335 syntax error — likely a BOOL or PIXELFORMATDESCRIPTOR
   type collision with the modern SDK.

Windows CI is currently skipped in ci-gtk3.yml (bug #618) until this is
resolved.

Approach:
Phase 1 — Fix include path so mswint.h is found (may resolve cascade errors)
Phase 2 — Audit wlib.h vs mswlib signature mismatches, update implementations
Phase 3 — Fix individual SDK incompatibilities (BC_HORZ, bitmap syntax)
Phase 4 — Re-enable Windows in ci-gtk3.yml and pass CI

Branch: bug-618-mswlib-msvc-porting

[Martin Fischer]

Duplicate see bug #612

[Martin Fischer]

Closed after CI was changed to use MSYS-mingw GTK3 packages