Fix the -geometry command line argument for Windows.

The command line argument -geometry WIDTHxHEIGHT±XOFF±YOFF
specifies a preferred size and location for the main window.
Currently, this is semi-broken on Windows. Indeed, only
specifying WIDTH and HEIGHT places the main window such that
the left and top borders are invisible such that the window cannot
be moved. Moreover, the XOFF and YOFF parts (when present) are
used to specify the distance of the window from the left and top
or right and bottom edges of the screen, when using '+' or '-',
respectively. However, -geometry 800x600-20-20, instead of placing
the window such that its bottom and right edges are at a distance
of 20 pixels from the corresponding screen edges, places the
window such that its left and top borders are out of the screen.
This is corrected by this commit.