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.
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msys2-mingw32