imgui: reorganize imgui library files to mirror what's in the imgui releases (#285)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Functionally, no change.

Move ImGui library files around so that they mirror how they are
structured in imgui releases. This will make it easier to migrate to a
future version of ImGui.

Add a handful of static asserts to check that we have the necessary
features enabled / disabled in imgui header.

## Compiling
👍 

## Testing
Tested 32-bit overlay, 64-bit overlay, TDJ, and UFC overlays, and
spicecfg.

Interesting finds during ImGui testing (not part of this PR, but for
future reference when upgrading imgui version):
* Updating dx9 backend to the newest causes TDJ to stop rendering the
overlay completely (fine in UFC or any other game)
* Omitting `IMGUI_USE_BGRA_PACKED_COLOR` causes spicecfg software
renderer to use the wrong color (RGB format parsing error probably)
* At some point - v1.90? the `About` and `Licenses` buttons in spicecfg
stop responding after an upgrade, maybe our improper usage of MenuItem?
* Using the docking branch of ImGui is not required but we should just
stick to it for multi-viewport functionality which we may want to use
later.

src/spice2x/CMakeLists.txt

@@ -211,7 +211,7 @@ add_compile_definitions(
 #set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DACIOEMU_LOG")
 
 # add project directory to include path so we can comfortably import
-include_directories(${spicetools_SOURCE_DIR})
+include_directories(${spicetools_SOURCE_DIR} ${spicetools_SOURCE_DIR}/external/imgui)
 
 # add external libraries
 add_subdirectory(external/fmt EXCLUDE_FROM_ALL)
@@ -516,7 +516,6 @@ set(SOURCE_FILES ${SOURCE_FILES}
         overlay/overlay.cpp
         overlay/window.cpp
         overlay/imgui/extensions.cpp
-        overlay/imgui/impl_dx9.cpp
         overlay/imgui/impl_spice.cpp
         overlay/imgui/impl_sw.cpp
         overlay/windows/acio_status_buffers.cpp

src/spice2x/external/imgui/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.9)
+cmake_minimum_required(VERSION 3.12)
 project(imgui CXX)
 
 # dear imgui uses the C++ virtual interfaces for DirectX
@@ -8,7 +8,8 @@ set(IMGUI_HEADERS
         imconfig.h
         imgui.h
         imgui_internal.h
-        imgui_stdlib.h
+        backends/imgui_impl_dx9.h
+        misc/cpp/imgui_stdlib.h
 )
 set(IMGUI_SOURCES
         imgui.cpp
@@ -16,7 +17,8 @@ set(IMGUI_SOURCES
         imgui_tables.cpp
         imgui_widgets.cpp
         imgui_demo.cpp
-        imgui_stdlib.cpp
+        backends/imgui_impl_dx9.cpp
+        misc/cpp/imgui_stdlib.cpp
 )
 
 add_library(imgui STATIC ${IMGUI_HEADERS} ${IMGUI_SOURCES})

src/spice2x/external/imgui/backends/imgui_impl_dx9.cpp

@@ -22,14 +22,14 @@
 //  2018-02-16: Misc: Obsoleted the io.RenderDrawListsFn callback and exposed ImGui_ImplDX9_RenderDrawData() in the .h file so you can call it yourself.
 //  2018-02-06: Misc: Removed call to ImGui::Shutdown() which is not available from 1.60 WIP, user needs to call CreateContext/DestroyContext themselves.
 
-#include "impl_dx9.h"
+#include "imgui_impl_dx9.h"
 
 #include <algorithm>
 
 // DirectX
 #include <d3d9.h>
 
-#include "external/imgui/imgui.h"
+#include "imgui.h"
 
 // allow std::min use
 #ifdef min

src/spice2x/external/imgui/backends/imgui_impl_dx9.h

@@ -11,7 +11,7 @@
 
 #pragma once
 
-#include "external/imgui/imgui.h"
+#include "imgui.h"
 
 struct IDirect3DDevice9;
 

src/spice2x/overlay/imgui/impl_spice.cpp

@@ -13,6 +13,14 @@
 #include "util/logging.h"
 #include "util/utils.h"
 
+#if !defined(IMGUI_ENABLE_WIN32_DEFAULT_IME_FUNCTIONS) || \
+    !defined(IMGUI_DISABLE_DEFAULT_ALLOCATORS) || \
+    !defined(IMGUI_USE_BGRA_PACKED_COLOR) || \
+    !defined(IMGUI_HAS_VIEWPORT) || \
+    !defined(IMGUI_HAS_DOCK)
+#error "fix imconfig.h after updating imgui version"
+#endif
+
 // state
 static HWND g_hWnd = nullptr;
 static INT64 g_Time = 0;

src/spice2x/overlay/overlay.cpp

@@ -11,10 +11,7 @@
 #include "util/resutils.h"
 #include "build/resource.h"
 
-#include "imgui/impl_dx9.h"
-#include "imgui/impl_spice.h"
-#include "imgui/impl_sw.h"
-#include "overlay/imgui/impl_dx9.h"
+#include "external/imgui/backends/imgui_impl_dx9.h"
 #include "overlay/imgui/impl_spice.h"
 #include "overlay/imgui/impl_sw.h"
 

src/spice2x/overlay/window.h

@@ -3,7 +3,7 @@
 #include <string>
 
 #include "external/imgui/imgui.h"
-#include "external/imgui/imgui_stdlib.h"
+#include "external/imgui/misc/cpp/imgui_stdlib.h"
 #include "overlay.h"
 
 namespace overlay {