If a window is configured with a fixed size (it's tiled, maximized, or
fullscreen), ignore any resize call that doesn't respect this. The set
size will instead be saved, when appropriate, so that the new size is
used when e.g. unmaximizing.
This makes it possible to call 'gtk_window_resize()' while the window is
maximized, without the window actually changing size until it's
unmaximized. Changing size to a non-maximized size is a violation of the
xdg-shell protocol.
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