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/opengl2_example/main.cpp

@@ -7,13 +7,14 @@
 // See imgui_impl_glfw.cpp for details.
 
 #include "imgui.h"
-#include "imgui_impl_glfw_gl2.h"
+#include "imgui_impl_glfw.h"
+#include "imgui_impl_opengl2.h"
 #include <stdio.h>
 #include <GLFW/glfw3.h>
 
 static void glfw_error_callback(int error, const char* description)
 {
-    fprintf(stderr, "Error %d: %s\n", error, description);
+    fprintf(stderr, "Glfw Error %d: %s\n", error, description);
 }
 
 int main(int, char**)
@@ -31,7 +32,10 @@ int main(int, char**)
     ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     //io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;  // Enable Keyboard Controls
-    ImGui_ImplGlfwGL2_Init(window, true);
+    //io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;   // Enable Gamepad Controls
+
+    ImGui_ImplGlfw_InitForOpenGL(window, true);
+    ImGui_ImplOpenGL2_Init();
 
     // Setup style
     ImGui::StyleColorsDark();
@@ -59,12 +63,17 @@ int main(int, char**)
     // Main loop
     while (!glfwWindowShouldClose(window))
     {
+        // Poll and handle events (inputs, window resize, etc.)
         // You can read the io.WantCaptureMouse, io.WantCaptureKeyboard flags to tell if dear imgui wants to use your inputs.
         // - When io.WantCaptureMouse is true, do not dispatch mouse input data to your main application.
         // - When io.WantCaptureKeyboard is true, do not dispatch keyboard input data to your main application.
         // Generally you may always pass all inputs to dear imgui, and hide them from your application based on those two flags.
         glfwPollEvents();
-        ImGui_ImplGlfwGL2_NewFrame();
+
+        // Start the ImGui frame
+        ImGui_ImplOpenGL2_NewFrame();
+        ImGui_ImplGlfw_NewFrame();
+        ImGui::NewFrame();
 
         // 1. Show a simple window.
         // Tip: if we don't call ImGui::Begin()/ImGui::End() the widgets automatically appears in a window called "Debug".
@@ -104,19 +113,22 @@ int main(int, char**)
         }
 
         // Rendering
+        ImGui::Render();
         int display_w, display_h;
         glfwGetFramebufferSize(window, &display_w, &display_h);
         glViewport(0, 0, display_w, display_h);
         glClearColor(clear_color.x, clear_color.y, clear_color.z, clear_color.w);
         glClear(GL_COLOR_BUFFER_BIT);
         //glUseProgram(0); // You may want this if using this code in an OpenGL 3+ context where shaders may be bound, but prefer using the GL3+ code.
-        ImGui::Render();
-        ImGui_ImplGlfwGL2_RenderDrawData(ImGui::GetDrawData());
+        ImGui_ImplOpenGL2_RenderDrawData(ImGui::GetDrawData());
+
+        glfwMakeContextCurrent(window);
         glfwSwapBuffers(window);
     }
 
     // Cleanup
-    ImGui_ImplGlfwGL2_Shutdown();
+    ImGui_ImplOpenGL2_Shutdown();
+    ImGui_ImplGlfw_Shutdown();
     ImGui::DestroyContext();
 
     glfwDestroyWindow(window);