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/overlay/overlay.h

@@ -10,11 +10,22 @@
 
 #include "external/imgui/imgui.h"
 
+#ifdef SPICE_D3D11
+// forward decls for D3D11 (avoid pulling d3d11.h into every TU)
+struct ID3D11Device;
+struct ID3D11DeviceContext;
+struct ID3D11RenderTargetView;
+struct IDXGISwapChain;
+#endif
+
 namespace overlay {
     class Window;
 
     enum class OverlayRenderer {
         D3D9,
+#ifdef SPICE_D3D11
+        D3D11,
+#endif
         SOFTWARE,
     };
 
@@ -62,6 +73,10 @@ namespace overlay {
         Window *window_log = nullptr;
 
         explicit SpiceOverlay(HWND hWnd, IDirect3D9 *d3d, IDirect3DDevice9 *device);
+#ifdef SPICE_D3D11
+        explicit SpiceOverlay(HWND hWnd, ID3D11Device *d3d11_device,
+                              ID3D11DeviceContext *d3d11_context, IDXGISwapChain *d3d11_swapchain);
+#endif
         explicit SpiceOverlay(HWND hWnd);
         ~SpiceOverlay();
 
@@ -92,6 +107,14 @@ namespace overlay {
         inline bool uses_device(IDirect3DDevice9 *other) {
             return this->device == other;
         }
+#ifdef SPICE_D3D11
+        inline bool uses_device(ID3D11Device *other) {
+            return this->d3d11_device == other;
+        }
+        inline bool uses_swapchain(IDXGISwapChain *other) {
+            return this->d3d11_swapchain == other;
+        }
+#endif
         inline IDirect3DDevice9 *get_device() {
             return this->device;
         }
@@ -124,6 +147,14 @@ namespace overlay {
         IDirect3D9 *d3d = nullptr;
         IDirect3DDevice9 *device = nullptr;
 
+#ifdef SPICE_D3D11
+        // D3D11
+        ID3D11Device *d3d11_device = nullptr;
+        ID3D11DeviceContext *d3d11_context = nullptr;
+        IDXGISwapChain *d3d11_swapchain = nullptr;
+        ID3D11RenderTargetView *d3d11_rtv = nullptr;
+#endif
+
         // software
         std::vector<uint32_t> pixel_data;
         size_t pixel_data_width = 0;
@@ -156,6 +187,10 @@ namespace overlay {
 
     // synchronized helpers
     void create_d3d9(HWND hWnd, IDirect3D9 *d3d, IDirect3DDevice9 *device);
+#ifdef SPICE_D3D11
+    void create_d3d11(HWND hWnd, ID3D11Device *device, ID3D11DeviceContext *context,
+                      IDXGISwapChain *swapchain);
+#endif
     void create_software(HWND hWnd);
     void destroy(HWND hWnd = nullptr);
 }