gtkwindow: ignore default size if there is a size request

gtkwindow: ignore default size if there is a size request

Some applications set both a default size on their gtk window and a size
request on the corresponding gtk widget.

Until now, the default size was ignored for fixed size windows, so this
had no effect and remained unnoticed, but with the recent change for
client-side decorations, the default size is now used even for fixed size
windows, which can cause the resulting fixed size window to be much
smaller than expected with the size request.

For fixed size windows, if we have both a size request and a default
size set, prefer the size request as before.

https://bugzilla.gnome.org/show_bug.cgi?id=763749