Initial commit.

Framework/external/imgui/backends/imgui_impl_opengl2.h

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+// dear imgui: Renderer Backend for OpenGL2 (legacy OpenGL, fixed pipeline)
+// This needs to be used along with a Platform Backend (e.g. GLFW, SDL, Win32, custom..)
+
+// Implemented features:
+//  [X] Renderer: User texture binding. Use 'GLuint' OpenGL texture identifier as void*/ImTextureID. Read the FAQ about ImTextureID!
+//  [X] Renderer: Multi-viewport support (multiple windows). Enable with 'io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable'.
+
+// You can use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
+// Prefer including the entire imgui/ repository into your project (either as a copy or as a submodule), and only build the backends you need.
+// Learn about Dear ImGui:
+// - FAQ                  https://dearimgui.com/faq
+// - Getting Started      https://dearimgui.com/getting-started
+// - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
+// - Introduction, links and more at the top of imgui.cpp
+
+// **DO NOT USE THIS CODE IF YOUR CODE/ENGINE IS USING MODERN OPENGL (SHADERS, VBO, VAO, etc.)**
+// **Prefer using the code in imgui_impl_opengl3.cpp**
+// This code is mostly provided as a reference to learn how ImGui integration works, because it is shorter to read.
+// If your code is using GL3+ context or any semi modern OpenGL calls, using this is likely to make everything more
+// complicated, will require your code to reset every single OpenGL attributes to their initial state, and might
+// confuse your GPU driver.
+// The GL2 code is unable to reset attributes or even call e.g. "glUseProgram(0)" because they don't exist in that API.
+
+#pragma once
+#include "imgui.h"      // IMGUI_IMPL_API
+#ifndef IMGUI_DISABLE
+
+IMGUI_IMPL_API bool     ImGui_ImplOpenGL2_Init();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL2_Shutdown();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL2_NewFrame();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL2_RenderDrawData(ImDrawData* draw_data);
+
+// Called by Init/NewFrame/Shutdown
+IMGUI_IMPL_API bool     ImGui_ImplOpenGL2_CreateFontsTexture();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL2_DestroyFontsTexture();
+IMGUI_IMPL_API bool     ImGui_ImplOpenGL2_CreateDeviceObjects();
+IMGUI_IMPL_API void     ImGui_ImplOpenGL2_DestroyDeviceObjects();
+
+#endif // #ifndef IMGUI_DISABLE