gitadora: add native dual fullscreen for Arena (#846)

## Link to GitHub Issue or related Pull Request, if one exists

Fixed https://github.com/spice2x/spice2x.github.io/issues/740

## Description of change

- Enable native D3D9 adapter-group dual fullscreen for the GITADORA
Arena model when the two-screen MAIN + SMALL configuration is selected
without windowed mode.
- Present MAIN and SMALL on their respective monitor heads while keeping
the invisible LEFT and RIGHT targets offscreen.
- Scope the new fake-swap-chain query behavior to the hidden GITADORA
two-head targets, preserving existing behavior for other games and
configurations.

The existing borderless-windowed two-screen path could not consistently
keep input and game timing synchronized and also reduced rendering
performance. Native dual fullscreen avoids that windowed composition
path.

## Testing

- Manually tested GITADORA Arena with separate MAIN and SMALL monitors
in fullscreen mode. Windowed mode still works if the user chooses to use
that.
- Confirmed both displays render correctly, touch input works on the
SMALL screen, gameplay input stays synchronized, and gameplay holds a
steady 60 FPS.
- Built `spicetools_spice64` at commit `e4a98e9` with the repository
Docker toolchain.
- Confirmed no new compiler warnings.
- Confirmed the forbidden static DLL import check passes.
- Confirmed the Windows 7 DLL compatibility check reports `All DLLs
OK!`.

---------

Co-authored-by: vgod <428979+vgod@users.noreply.github.com>

src/spice2x/games/gitadora/gitadora.h

@@ -19,6 +19,7 @@ namespace games::gitadora {
     extern std::optional<std::string> SUBSCREEN_OVERLAY_SIZE;
     extern std::optional<socd::SocdAlgorithm> PICK_ALGO;
     extern std::optional<uint8_t> ARENA_WINDOW_COUNT;
+    extern bool ARENA_TWO_HEAD_EXCLUSIVE;
     extern std::optional<std::string> ASIO_DRIVER;
     extern bool ALLOW_REALTEK_AUDIO;