Add ImGui source files.

Signed-off-by: aixxe <me@aixxe.net>

include/imgui/imgui_impl_sdl.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <GL/gl.h>
+#include <SDL2/SDL.h>
+
+struct SDL_Window;
+typedef union SDL_Event SDL_Event;
+
+IMGUI_API bool ImGui_ImplSdl_Init(SDL_Window* window);
+IMGUI_API void ImGui_ImplSdl_Shutdown();
+IMGUI_API void ImGui_ImplSdl_NewFrame(SDL_Window* window);
+IMGUI_API bool ImGui_ImplSdl_ProcessEvent(SDL_Event* event);
+
+IMGUI_API void ImGui_ImplSdl_InvalidateDeviceObjects();
+IMGUI_API bool ImGui_ImplSdl_CreateDeviceObjects();