Implement ImGui.

UnleashedRecomp/ui/window.cpp

@@ -7,6 +7,9 @@ bool m_isFullscreenKeyReleased = true;
 
 int Window_OnSDLEvent(void*, SDL_Event* event)
 {
+    if (ImGui::GetIO().BackendPlatformUserData != nullptr)
+        ImGui_ImplSDL2_ProcessEvent(event);
+
     switch (event->type)
     {
         case SDL_QUIT:

UnleashedRecomp/ui/window.h

@@ -1,6 +1,5 @@
 #pragma once
 
-#include <SDL.h>
 #include "res/icon.h"
 #include "ui/window_events.h"
 #include "config.h"