ui/app: introduce DestroyEvent for ending the event loop

Replace the StageDead stage with DestroyEvent dedicated to ending
the event loop and, for premature window closes, the error.

Drop the error return from NewWindow; any errors in window creation
will appear as an immediate DestroyEvent with its Err field set.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/os_wayland.go

@@ -113,6 +113,7 @@ type window struct {
 	lastTouch f32.Point
 
 	stage             Stage
+	dead              bool
 	lastFrameCallback *C.struct_wl_callback
 
 	mu        sync.Mutex
@@ -308,7 +309,8 @@ func gio_onXdgSurfaceConfigure(data unsafe.Pointer, wmSurf *C.struct_xdg_surface
 //export gio_onToplevelClose
 func gio_onToplevelClose(data unsafe.Pointer, topLvl *C.struct_xdg_toplevel) {
 	w := winMap[topLvl]
-	w.setStage(StageDead)
+	w.dead = true
+	w.w.event(DestroyEvent{})
 }
 
 //export gio_onToplevelConfigure
@@ -769,7 +771,7 @@ loop:
 		if ret := C.wl_display_flush(conn.disp); ret < 0 {
 			break
 		}
-		if w.stage == StageDead {
+		if w.dead {
 			break
 		}
 		// Clear poll events.