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/directx11_example/imgui_impl_dx11.cpp

@@ -252,6 +252,9 @@ static bool IsAnyMouseButtonDown()
 // PS: We treat DBLCLK messages as regular mouse down messages, so this code will work on windows classes that have the CS_DBLCLKS flag set. Our own example app code doesn't set this flag.
 IMGUI_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 {
+    if (ImGui::GetCurrentContext() == NULL)
+        return 0;
+
     ImGuiIO& io = ImGui::GetIO();
     switch (msg)
     {
@@ -574,7 +577,6 @@ bool    ImGui_ImplDX11_Init(void* hwnd, ID3D11Device* device, ID3D11DeviceContex
 void ImGui_ImplDX11_Shutdown()
 {
     ImGui_ImplDX11_InvalidateDeviceObjects();
-    ImGui::Shutdown();
     g_pd3dDevice = NULL;
     g_pd3dDeviceContext = NULL;
     g_hWnd = (HWND)0;

examples/directx11_example/main.cpp

@@ -123,6 +123,7 @@ int main(int, char**)
     UpdateWindow(hwnd);
 
     // Setup ImGui binding
+    ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     ImGui_ImplDX11_Init(hwnd, g_pd3dDevice, g_pd3dDeviceContext);
     //io.NavFlags |= ImGuiNavFlags_EnableKeyboard;  // Enable Keyboard Controls
@@ -214,6 +215,8 @@ int main(int, char**)
     }
 
     ImGui_ImplDX11_Shutdown();
+    ImGui::DestroyContext();
+
     CleanupDeviceD3D();
     UnregisterClass(_T("ImGui Example"), wc.hInstance);