Possible fix for taskbar hiding issues in fullscreen

I just found out about MarkFullscreenWindow. Though I couldn't reproduce #999, I hope this fixes it, since [Firefox also uses it][1] to solve similar issues.

This also makes OLE initialization more robust, since it relies on the fact that initializing OLE also initializes COM. Both S_OK and S_FALSE are valid successful return values from OleInitialize.

[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=617052