Big example binding refactor (manually imported from Viewport branch, stripped out of Viewport code). (#1870)

Read examples/README.txt for some details.
ImDrawData: Added DisplayPos, DisplaySize fields honored by all backends (not strictly necessary to honor just now, but doing it to be ahead)

examples/imgui_impl_glfw.h

@@ -0,0 +1,30 @@
+// ImGui Platform Binding for: GLFW
+// This needs to be used along with a Renderer (e.g. OpenGL3, Vulkan..)
+// (Info: GLFW is a cross-platform general purpose library for handling windows, inputs, OpenGL/Vulkan graphics context creation, etc.)
+
+// Implemented features:
+//  [X] Gamepad navigation mapping. Enable with 'io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad'.
+
+// You can copy and use unmodified imgui_impl_* files in your project. See main.cpp for an example of using this.
+// If you use this binding you'll need to call 4 functions: ImGui_ImplXXXX_Init(), ImGui_ImplXXXX_NewFrame(), ImGui::Render() and ImGui_ImplXXXX_Shutdown().
+// If you are new to ImGui, see examples/README.txt and documentation at the top of imgui.cpp.
+// https://github.com/ocornut/imgui
+
+// About GLSL version:
+// The 'glsl_version' initialization parameter defaults to "#version 150" if NULL.
+// Only override if your GL version doesn't handle this GLSL version. Keep NULL if unsure!
+
+struct GLFWwindow;
+
+IMGUI_API bool        ImGui_ImplGlfw_InitForOpenGL(GLFWwindow* window, bool install_callbacks);
+IMGUI_API bool        ImGui_ImplGlfw_InitForVulkan(GLFWwindow* window, bool install_callbacks);
+IMGUI_API void        ImGui_ImplGlfw_Shutdown();
+IMGUI_API void        ImGui_ImplGlfw_NewFrame();
+
+// GLFW callbacks (installed by default if you enable 'install_callbacks' during initialization)
+// Provided here if you want to chain callbacks.
+// You can also handle inputs yourself and use those as a reference.
+IMGUI_API void        ImGui_ImplGlfw_MouseButtonCallback(GLFWwindow* window, int button, int action, int mods);
+IMGUI_API void        ImGui_ImplGlfw_ScrollCallback(GLFWwindow* window, double xoffset, double yoffset);
+IMGUI_API void        ImGui_ImplGlfw_KeyCallback(GLFWwindow* window, int key, int scancode, int action, int mods);
+IMGUI_API void        ImGui_ImplGlfw_CharCallback(GLFWwindow* window, unsigned int c);