wayland: don't use wl_display_dispatch()

wl_display_dispatch() calls poll(), which is unnecessary since we
already know the FD is readable.

Use the more lower level wl_display_read_events() +
wl_display_dispatch_pending().

These require wl_display_prepare_read() to have been called.

The idea is to call wl_display_prepare_read() **before** calling
poll().

Thus, we do this more or less last in wayl_init(), and at the **end**
of the FDM handler.

However, having taking this lock also means we no longer can call
wl_display_roundtrip() directly (it will hang).

So, add a wrapper, wayl_roundtrip(), that cancels the read intent,
does the roundtrip, and then re-acquires the read intent.

terminal.c

@@ -659,9 +659,7 @@ term_init(const struct config *conf, struct fdm *fdm, struct wayland *wayl,
 
     /* Let the Wayland backend know we exist */
     tll_push_back(wayl->terms, term);
-
-    /* Roundtrip to ensure the wayland window have been configured */
-    wl_display_roundtrip(term->wl->display);
+    wayl_roundtrip(term->wl);
 
     term_set_window_title(term, "foot");