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_backend.h

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "overlay/overlay.h"
+
+void graphics_d3d11_init();
+void graphics_d3d11_shutdown();
+
+#ifdef SPICE_D3D11
+
+struct ID3D11Device;
+struct ID3D11DeviceContext;
+struct ID3D11RenderTargetView;
+struct IDXGISwapChain;
+
+namespace overlay::d3d11 {
+
+    void render(ID3D11Device *device,
+                ID3D11DeviceContext *context,
+                IDXGISwapChain *swapchain,
+                ID3D11RenderTargetView **rtv);
+
+}
+
+#endif