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/opengl3_example/imgui_impl_glfw_gl3.cpp

@@ -362,7 +362,6 @@ bool    ImGui_ImplGlfwGL3_Init(GLFWwindow* window, bool install_callbacks)
 void ImGui_ImplGlfwGL3_Shutdown()
 {
     ImGui_ImplGlfwGL3_InvalidateDeviceObjects();
-    ImGui::Shutdown();
 }
 
 void ImGui_ImplGlfwGL3_NewFrame()

examples/opengl3_example/main.cpp

@@ -32,6 +32,7 @@ int main(int, char**)
     gl3wInit();
 
     // Setup ImGui binding
+    ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     ImGui_ImplGlfwGL3_Init(window, true);
     //io.NavFlags |= ImGuiNavFlags_EnableKeyboard;  // Enable Keyboard Controls
@@ -119,6 +120,7 @@ int main(int, char**)
 
     // Cleanup
     ImGui_ImplGlfwGL3_Shutdown();
+    ImGui::DestroyContext();
     glfwTerminate();
 
     return 0;