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/launcher/shutdown.cpp

@@ -8,6 +8,7 @@
 #include "rawinput/rawinput.h"
 #include "hooks/audio/audio.h"
 #include "hooks/graphics/graphics.h"
+#include "hooks/graphics/backends/d3d11/d3d11_backend.h"
 #include "util/deferlog.h"
 #include "util/logging.h"
 
@@ -26,6 +27,10 @@ namespace launcher {
 
         sdk::fini_sdk_modules();
 
+        // stop dx11 background workers (poll thread, LDR notification)
+        // before anything else, so they can't race against the teardown.
+        graphics_d3d11_shutdown();
+
         // reset monitor settings
         reset_monitor_on_exit();