Use correct type for window_handler and cast to correct type in MinGW
environment. Based on
https://gstreamer.freedesktop.org/documentation/video/gstvideooverlay.html?gi-language=c#gstvideooverlay-and-gtk

Fixes issue #18.

Compilation errors fixed:
```
[...]
../../examples/media_player_gtkmm/player_window.cc:464:2: error: unterminated argument list invoking macro "GDK_WINDOW_HWND"
  464 | }
      |  ^
../../examples/media_player_gtkmm/player_window.cc: In member function 'void PlayerWindow::on_video_area_realize()':
../../examples/media_player_gtkmm/player_window.cc:141:20: error: 'GDK_WINDOW_HWND' was not declared in this scope
  141 |   window_handler = GDK_WINDOW_HWND ((m_video_area.get_window()->gobj());
      |                    ^~~~~~~~~~~~~~~
../../examples/media_player_gtkmm/player_window.cc:141:35: error: expected '}' at end of input
  141 |   window_handler = GDK_WINDOW_HWND ((m_video_area.get_window()->gobj());
      |                                   ^
../../examples/m...