app: [macOS] panic if Main is not called from the main goroutine

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

app/os_darwin.go

@@ -75,9 +75,13 @@ var displayLinks sync.Map
 
 var mainFuncs = make(chan func(), 1)
 
+func isMainThread() bool {
+	return bool(C.isMainThread())
+}
+
 // runOnMain runs the function on the main thread.
 func runOnMain(f func()) {
-	if C.isMainThread() {
+	if isMainThread() {
 		f()
 		return
 	}

app/os_macos.go

@@ -976,6 +976,9 @@ func (w *window) init() error {
 }
 
 func osMain() {
+	if !isMainThread() {
+		panic("app.Main must run on the main goroutine")
+	}
 	C.gio_main()
 }