ebiten: Make NewImage/NewImageFromImage panic when RunGame finishes

Closes #1149

image.go

@@ -748,7 +748,12 @@ func (i *Image) ReplacePixels(pixels []byte) {
 // NewImage returns an empty image.
 //
 // If width or height is less than 1 or more than device-dependent maximum size, NewImage panics.
+//
+// NewImage panics if RunGame already finishes.
 func NewImage(width, height int) *Image {
+	if isRunGameEnded() {
+		panic(fmt.Sprintf("ebiten: NewImage cannot be called after RunGame finishes"))
+	}
 	if width <= 0 {
 		panic(fmt.Sprintf("ebiten: width at NewImage must be positive but %d", width))
 	}
@@ -766,9 +771,14 @@ func NewImage(width, height int) *Image {
 // NewImageFromImage creates a new image with the given image (source).
 //
 // If source's width or height is less than 1 or more than device-dependent maximum size, NewImageFromImage panics.
+//
+// NewImageFromImage panics if RunGame already finishes.
 func NewImageFromImage(source image.Image) *Image {
-	size := source.Bounds().Size()
+	if isRunGameEnded() {
+		panic(fmt.Sprintf("ebiten: NewImage cannot be called after RunGame finishes"))
+	}
 
+	size := source.Bounds().Size()
 	width, height := size.X, size.Y
 	if width <= 0 {
 		panic(fmt.Sprintf("ebiten: source width at NewImageFromImage must be positive but %d", width))

run.go

@@ -75,6 +75,7 @@ func CurrentFPS() float64 {
 
 var (
 	isScreenClearedEveryFrame = int32(1)
+	isRunGameEnded_           = int32(0)
 	currentMaxTPS             = int32(DefaultTPS)
 )
 
@@ -162,6 +163,8 @@ func (i *imageDumperGame) Layout(outsideWidth, outsideHeight int) (screenWidth,
 //
 // Don't call RunGame twice or more in one process.
 func RunGame(game Game) error {
+	defer atomic.StoreInt32(&isRunGameEnded_, 1)
+
 	fixWindowPosition(WindowSize())
 	theUIContext.set(&imageDumperGame{
 		game: game,
@@ -175,6 +178,10 @@ func RunGame(game Game) error {
 	return nil
 }
 
+func isRunGameEnded() bool {
+	return atomic.LoadInt32(&isRunGameEnded_) != 0
+}
+
 // RunGameWithoutMainLoop runs the game, but doesn't call the loop on the main (UI) thread.
 // Different from Run, RunGameWithoutMainLoop returns immediately.
 //