window-wayland: Don't use grab op for some other window when resizing

When resizing a window interactively, we'll set a grab operation and a grab window, among other things. If we're resizing (including setting initial size, i.e. mapping) another window, that didn't change position, don't use the gravity of the grab operation when resizing our own window.

This fixes an issue with jumpy popup position when moving a previously mapped gtk popover.

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