Overrides previous, Windows-only hack used for the same purpose.
After initial window position, OS or window manager will take care of
emplacing window - this change takes care only of situation, when there
is NO initial window position.
Additionally, avoid re-setting the window size on every SetSDLWindowMode
call. This prevents a number of issues with the window being set to
incorrect size while switching between fullscreen and window.
Overally this change takes care of number of small issues happening on
macOS, Windows (without need for an ifdef), and Linux with KDE and MATE.
Also makes the behaviour more consistent for Gnome when starting dosbox
via XWayland.
e4b19ccd37
