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/graphics.cpp

@@ -18,6 +18,7 @@
 #include "games/iidx/iidx.h"
 #include "games/popn/popn.h"
 #include "hooks/graphics/backends/d3d9/d3d9_backend.h"
+#include "hooks/graphics/backends/d3d11/d3d11_backend.h"
 #include "launcher/shutdown.h"
 #include "overlay/overlay.h"
 #include "touch/touch.h"
@@ -859,6 +860,7 @@ void graphics_init() {
 
     // init backends
     graphics_d3d9_init();
+    graphics_d3d11_init();
 
     // general hooks
     ChangeDisplaySettingsA_orig = detour::iat_try("ChangeDisplaySettingsA", ChangeDisplaySettingsA_hook);