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.
This does not, in anyway, modifies the ImGui functionality but only shows how it can be used to set up "independent" windows.

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