backend/{x11, wayland}: commit new custom mode on window configure

Fixes: #3418

From my understanding this is how backends acknowledge new modes: the compositor uses wlr_output_set_mode which will set WLR_OUTPUT_STATE_MODE. The backend will respond to this commit event and then use wlr_output_update_mode. Unfortunately if either the x11 or wayland backends get configured (resized) they will skip the commit part and jump straight to updating the sizes. Fix this my making the configures go through a commit first.

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