GDK W32: Handle maximizing correctly for small primary monitors

GDK W32: Handle maximizing correctly for small primary monitors

When primary monitor is smaller than the actual monitor on which the
window is being maximized, the WM will do widnow size adjustments
that will completely screw the window size if we try to make it
smaller than 100% fullscreen (to account for taskbar size, for example).

Fix this by overriding maximized window size during WM_WINDOWPOSCHANGING.

https://bugzilla.gnome.org/show_bug.cgi?id=775808