Skip stale frame extents when maximizing/fullscreening

[gene1wood]

PR #2441 added the client's advertised _GTK_FRAME_EXTENTS to the geometry sent in configure_with_sync() so XWayland CSD windows (e.g. Firefox) wouldn't shrink. But set_maximized()/set_geometry() are called back-to-back, and Smithay's internal maximized/fullscreen state updates synchronously while the client's _GTK_FRAME_EXTENTS does not update until it processes the configure. This meant the window was briefly configured larger than the output using its old, pre-maximize shadow extents, leaving it jutting past the screen edge until an unrelated reconfigure (e.g. focus change on alt-tab) caught up with the client's now-zeroed extents.

GTK clients always drop their CSD shadows when maximized or fullscreen, so once is_maximized()/is_fullscreen() report true, skip frame_extents() and use zero instead of waiting for the client to confirm it.

Fixes #2503

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