Merge branch 'context'

# Conflicts:
#	examples/allegro5_example/main.cpp
#	examples/directx10_example/main.cpp
#	examples/directx11_example/main.cpp
#	examples/directx9_example/main.cpp
#	examples/marmalade_example/main.cpp
#	examples/opengl2_example/main.cpp
#	examples/opengl3_example/main.cpp
#	examples/sdl_opengl2_example/main.cpp
#	examples/sdl_opengl3_example/main.cpp
#	examples/vulkan_example/main.cpp

examples/sdl_opengl2_example/imgui_impl_sdl_gl2.cpp

@@ -250,7 +250,6 @@ bool    ImGui_ImplSdlGL2_Init(SDL_Window* window)
 void ImGui_ImplSdlGL2_Shutdown()
 {
     ImGui_ImplSdlGL2_InvalidateDeviceObjects();
-    ImGui::Shutdown();
 }
 
 void ImGui_ImplSdlGL2_NewFrame(SDL_Window *window)

examples/sdl_opengl2_example/main.cpp

@@ -34,6 +34,7 @@ int main(int, char**)
     SDL_GL_SetSwapInterval(1); // Enable vsync
 
     // Setup ImGui binding
+    ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     ImGui_ImplSdlGL2_Init(window);
     //io.NavFlags |= ImGuiNavFlags_EnableKeyboard;  // Enable Keyboard Controls
@@ -126,6 +127,8 @@ int main(int, char**)
 
     // Cleanup
     ImGui_ImplSdlGL2_Shutdown();
+    ImGui::DestroyContext();
+
     SDL_GL_DeleteContext(glcontext);
     SDL_DestroyWindow(window);
     SDL_Quit();