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/sdl_opengl2_example/main.cpp

@@ -7,7 +7,8 @@
 // See imgui_impl_sdl.cpp for details.
 
 #include "imgui.h"
-#include "imgui_impl_sdl_gl2.h"
+#include "imgui_impl_sdl2.h"
+#include "imgui_impl_opengl2.h"
 #include <stdio.h>
 #include <SDL.h>
 #include <SDL_opengl.h>
@@ -38,7 +39,9 @@ int main(int, char**)
     ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     //io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;  // Enable Keyboard Controls
-    ImGui_ImplSdlGL2_Init(window);
+
+    ImGui_ImplSDL2_InitForOpenGL(window, gl_context);
+    ImGui_ImplOpenGL2_Init();
 
     // Setup style
     ImGui::StyleColorsDark();
@@ -67,6 +70,7 @@ int main(int, char**)
     bool done = false;
     while (!done)
     {
+        // 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.
@@ -74,11 +78,15 @@ int main(int, char**)
         SDL_Event event;
         while (SDL_PollEvent(&event))
         {
-            ImGui_ImplSdlGL2_ProcessEvent(&event);
+            ImGui_ImplSDL2_ProcessEvent(&event);
             if (event.type == SDL_QUIT)
                 done = true;
         }
-        ImGui_ImplSdlGL2_NewFrame(window);
+
+        // Start the ImGui frame
+        ImGui_ImplOpenGL2_NewFrame();
+        ImGui_ImplSDL2_NewFrame(window);
+        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".
@@ -118,17 +126,18 @@ int main(int, char**)
         }
 
         // Rendering
-        glViewport(0, 0, (int)ImGui::GetIO().DisplaySize.x, (int)ImGui::GetIO().DisplaySize.y);
+        ImGui::Render();
+        glViewport(0, 0, (int)io.DisplaySize.x, (int)io.DisplaySize.y);
         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
-        ImGui::Render();
-        ImGui_ImplSdlGL2_RenderDrawData(ImGui::GetDrawData());
+        ImGui_ImplOpenGL2_RenderDrawData(ImGui::GetDrawData());
         SDL_GL_SwapWindow(window);
     }
 
     // Cleanup
-    ImGui_ImplSdlGL2_Shutdown();
+    ImGui_ImplOpenGL2_Shutdown();
+    ImGui_ImplSDL2_Shutdown();
     ImGui::DestroyContext();
 
     SDL_GL_DeleteContext(gl_context);