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/directx9_example/imgui_impl_dx9.cpp

@@ -191,6 +191,9 @@ 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)
     {
@@ -291,7 +294,6 @@ bool    ImGui_ImplDX9_Init(void* hwnd, IDirect3DDevice9* device)
 void ImGui_ImplDX9_Shutdown()
 {
     ImGui_ImplDX9_InvalidateDeviceObjects();
-    ImGui::Shutdown();
     g_pd3dDevice = NULL;
     g_hWnd = 0;
 }

examples/directx9_example/main.cpp

@@ -75,6 +75,7 @@ int main(int, char**)
     }
 
     // Setup ImGui binding
+    ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     ImGui_ImplDX9_Init(hwnd, g_pd3dDevice);
     //io.NavFlags |= ImGuiNavFlags_EnableKeyboard;  // Enable Keyboard Controls
@@ -182,6 +183,8 @@ int main(int, char**)
     }
 
     ImGui_ImplDX9_Shutdown();
+    ImGui::DestroyContext();
+
     if (g_pd3dDevice) g_pd3dDevice->Release();
     if (pD3D) pD3D->Release();
     UnregisterClass(_T("ImGui Example"), wc.hInstance);