render: throttle xcursor updates

With a bad behaving client (e.g. 'less' with mouse support enabled),
we can end up with a *lot* of xcursor updates (so much, that we
flooded the wayland socket before we implemented a blocking
wayl_flush()).

Since there's little point in updating the cursor more than once per
frame interval, use frame callbacks to throttle the updates.

This works more or lesslike normal terminal refreshes:

render_xcursor_set() stores the last terminal (window) that had (and
updated) the cursor.

The renderer's FDM hook checks if we have such a pending terminal set,
and if so, tries to refresh the cursor.

This is done by first checking if we're already waiting for a callback
from a previous cursor update, and if so we do nothing; the callback
will update the cursor for the next frame. If we're *not* already
waiting for a callback, we update the cursor immediately.

terminal.c

@@ -1674,7 +1674,7 @@ term_xcursor_update(struct terminal *term)
         selection_enabled(term) ? XCURSOR_TEXT :
         XCURSOR_HAND2;
 
-    wayl_cursor_set(term->wl, term);
+    render_xcursor_set(term);
 }
 
 void