Add `emulateIndependentWindows` flag to DirectX example

If the flag is set, the example tries to behave as if the ImGui
windows were first-class citizens in OS windowing.
The current approach has a few limitations:
 - framerate is significantly lower than in the original example
 - mouse movements that start inside the window cannot extend to the outside
   it, that is, resizing, moving and scrolling does not work too good
 - the process is not closed when all of the windows are closed