gitadora: (arena model) booting fullscreen with one monitor (#696)

## Link to GitHub Issue or related Pull Request, if one exists
#477 

## Description of change
By default the game needs 4 monitors.

With the new option to disable subscreens, you can now boot fullscreen
with a single 4K monitor, with subscreen overlay enabled. Side monitors
are not shown in the overlay; only the touch screen is emulated.

## Future work

- [ ] custom resolution (-forceres) doesn't work properly (hooking
ResetEx does allow the game to boot but will render incorrectly;
probably needs hex edits)
- [x] enable this automatically if the user has fewer than 4 monitors
- [ ] ability to boot with a 4k main screen + small subscreen (with no
side screens which do nothing gameplay wise), but this whole display
emulation is a nightmare to work with & test thoroughly so it'll have to
be in the future.


## Testing
Checklist:

- [x] fs on 1 monitor with option set
- [x] fs on 2 monitor with option set
- [x] windowed - default (4 windows, no overlay)
- [x] windowed - with option set (1 window, overlay)
- [x] fs with 4 monitors?
- [x] check popn hc for regression

src/spice2x/hooks/graphics/backends/d3d9/d3d9_device.h

@@ -204,7 +204,8 @@ struct WrappedIDirect3DDevice9 : IDirect3DDevice9Ex {
     std::atomic_ulong refs = 1;
 
     WrappedIDirect3DSwapChain9 *main_swapchain = nullptr;
-    WrappedIDirect3DSwapChain9 *sub_swapchain = nullptr;
-    FakeIDirect3DSwapChain9 *fake_sub_swapchain = nullptr;
+    WrappedIDirect3DSwapChain9 *sub_swapchain[3] = { nullptr, nullptr, nullptr };
+    FakeIDirect3DSwapChain9 *fake_sub_swapchain[3] = { nullptr, nullptr, nullptr };
+
     IDirect3DVertexShader9 *vertex_shader = nullptr;
 };