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/external/imgui/CMakeLists.txt

@@ -21,5 +21,12 @@ set(IMGUI_SOURCES
         misc/cpp/imgui_stdlib.cpp
 )
 
+# spice2x: DX11 backend is only built for non-XP toolchains. WinXP lacks
+# D3DCOMPILER_47.dll, and including this TU would pull in D3DCompile imports.
+if(NOT SPICE_XP)
+    list(APPEND IMGUI_HEADERS backends/imgui_impl_dx11.h)
+    list(APPEND IMGUI_SOURCES backends/imgui_impl_dx11.cpp)
+endif()
+
 add_library(imgui STATIC ${IMGUI_HEADERS} ${IMGUI_SOURCES})
 target_include_directories(imgui PRIVATE ${PROJECT_SOURCE_DIR})