swayidle: fix busy loop on writable FD

The wl_event_source_fd_update docs say:

> File descriptors are usually writable to begin with, so they do not need to
> be polled for writable until a write actually fails. When a write fails,
> the event mask can be changed to poll for readable and writable, delivering
> a dispatch callback when it is possible to write more. Once all data has
> been written, the mask can be changed to poll only for readable to avoid
> busy-looping on dispatch.

So we should only poll for WL_EVENT_WRITABLE if a write fails. I'm not yet sure
how to do this properly and Weston doesn't do it, so in the meantime I'll just
fix the busy loop. I'll ask them too.

Fixes https://github.com/swaywm/sway/issues/3190

swayidle/main.c

@@ -192,8 +192,7 @@ static void setup_sleep_listener(void) {
 	acquire_sleep_lock();
 
 	struct wl_event_source *source = wl_event_loop_add_fd(state.event_loop,
-		sd_bus_get_fd(bus), WL_EVENT_READABLE | WL_EVENT_WRITABLE,
-		dbus_event, bus);
+		sd_bus_get_fd(bus), WL_EVENT_READABLE, dbus_event, bus);
 	wl_event_source_check(source);
 }
 #endif
@@ -401,10 +400,12 @@ static int display_event(int fd, uint32_t mask, void *data) {
 		count = wl_display_dispatch_pending(state.display);
 		wl_display_flush(state.display);
 	}
+
 	if (count < 0) {
 		wlr_log_errno(WLR_ERROR, "wl_display_dispatch failed, exiting");
 		sway_terminate(0);
 	}
+
 	return count;
 }
 
@@ -462,7 +463,7 @@ int main(int argc, char *argv[]) {
 	wl_display_roundtrip(state.display);
 
 	struct wl_event_source *source = wl_event_loop_add_fd(state.event_loop,
-		wl_display_get_fd(state.display), WL_EVENT_READABLE | WL_EVENT_WRITABLE,
+		wl_display_get_fd(state.display), WL_EVENT_READABLE,
 		display_event, NULL);
 	wl_event_source_check(source);