overlay: implement dx11 backend (#707)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #134 

## Description of change
Add ImGui DX11 backend implementation, and integrate into the spice
overlay.

The tricky part is that some of the Unity games:

1. Launch multiple windows, and
2. Can be resized (even full screen games launch at a certain resolution
and then expand to fit desktop resolution)
3. lazy load DX11 DLLs

This PR also adds screenshot functionality, but screen resize is not
implemented.

## Testing

Seems to be working for most games. Need to do final tests for:

- [x] CCJ (attract movie plays as well)
- [x] Busou Shinki (title movie plays)
- [x] MFG
- [x] QuizKnock
- [x] Polaris Chord
- [x] check dx9 for regression

src/spice2x/hooks/graphics/backends/d3d11/d3d11_internal.h

@@ -0,0 +1,59 @@
+#pragma once
+
+// internal glue for the dx11 backend. all symbols gated on SPICE_D3D11.
+
+#include "overlay/overlay.h"
+
+#ifdef SPICE_D3D11
+
+#include <memory>
+
+#include "util/detour.h"
+#include "util/logging.h"
+
+struct HWND__; typedef HWND__ *HWND;
+struct IUnknown;
+struct IDXGISwapChain;
+
+namespace d3d11_hooks {
+
+    void install_swapchain_hooks(IDXGISwapChain *swapchain);
+    void install_factory_hooks(IUnknown *factory);
+    void try_capture_vtables();
+
+    // first non-null swapchain HWND wins; later ones (sub-screens, IME
+    // helpers) are ignored. the dummy capture window is exempted via
+    // ignore_hwnd.
+    void note_main_hwnd(HWND hwnd);
+    HWND main_hwnd();
+    void ignore_hwnd(HWND hwnd);
+
+    // capture backbuffer to PNG if a screenshot was requested.
+    void try_screenshot(IDXGISwapChain *swapchain);
+
+    // trampoline a virtual method by vtable index. on failure *orig is null.
+    inline bool hook_vtbl(void *iface, size_t index,
+                          void *hook, void **orig, const char *name)
+    {
+        void **vtbl = *reinterpret_cast<void ***>(iface);
+        void *target = vtbl[index];
+        // trampoline_try reads *orig before overwriting it.
+        *orig = target;
+        if (!detour::trampoline_try(target, hook, orig)) {
+            *orig = nullptr;
+            log_warning("graphics::d3d11", "failed to hook {}", name);
+            return false;
+        }
+        log_info("graphics::d3d11", "hooked {}", name);
+        return true;
+    }
+
+    // minimal COM RAII used by capture / screenshot paths.
+    struct com_release {
+        void operator()(IUnknown *p) const { if (p) p->Release(); }
+    };
+    template<typename T> using com_ptr = std::unique_ptr<T, com_release>;
+
+}
+
+#endif