backend/wayland: avoid nested calls to wl_display_dispatch

See #3920 (closed) for motivation. This MR is an extension of !4905 (closed); it replaces both wl_display_dispatch() calls in backend/wayland/output.c with calls to wl_display_dispatch_queue(), that only process the events for the specific objects that are being waited for.

The wayland backend logic is modified so that, when waiting for an xdg_surface.configure event, the call to wlr_output_send_request_state() is delayed until after the wl_display_dispatch_queue() loop ends: this ensures that it will only happen once, even if multiple configure events are received.

Also, wlr_wl_output_create() now blocks until the first configure is done.

Note: it is still possible to recursively invoke output_commit(), because when the output is mapped after being unmapped, wlr_output_send_request_state() gets called inside output_commit(), and emits a signal whose listeners could call output_commit(). I do not currently know of a good way to solve this, so I've left a TODO in the code. (I'm not marking this as a draft, because I think this MR is still a strict improvement.)

This involves parts of wlroots that I'm not very familiar with, so it's possible I've made a mistake somewhere.