gdkwindow-win32.c: Set WS_BORDER for fullscreen GL windows if requested
With some GL drivers, it may be the case that menus are not shown correctly in fullscreen GL windows because DWM is deactivated in the process. Force WS_BORDER to be applied to the fullscreen GL window so that we have a small 1px border when needed (by setting an envvar), so that DWM does not get deactivated, hence enabling the menus to show. Also, when we force WS_BORDER to be applied in this situation, we also deliberately place the window just outside the top lefthand corner of the screen by 1px and make the window 1px larger than the screen size, so that we effectively hide the 1px border from view. Fixes issue #1702.