image: Add image.Dipose and image.IsDisposed

graphicscontext.go

@@ -61,10 +61,10 @@ func (c *graphicsContext) postUpdate() error {
 
 func (c *graphicsContext) setSize(screenWidth, screenHeight, screenScale int) error {
 	if c.defaultRenderTarget != nil {
-		c.defaultRenderTarget.dispose()
+		c.defaultRenderTarget.Dispose()
 	}
 	if c.screen != nil {
-		c.screen.dispose()
+		c.screen.Dispose()
 	}
 
 	var err error

image.go

@@ -128,18 +128,26 @@ func (i *Image) At(x, y int) color.Color {
 	return color.RGBA{r, g, b, a}
 }
 
-func (i *Image) dispose() {
+// Dispose disposes the image data. After disposing, the image becomes invalid.
+// This is useful to save memory.
+func (i *Image) Dispose() {
+	if i.isDisposed() {
+		panic("the image is already disposed")
+	}
 	useGLContext(func(c *opengl.Context) {
-		if i.framebuffer != nil {
 		i.framebuffer.Dispose(c)
-		}
-		if i.texture != nil {
+		i.framebuffer = nil
 		i.texture.Dispose(c)
-		}
+		i.texture = nil
 	})
 	i.pixels = nil
 }
 
+// IsDisposed returns a boolean indicating wheather the image is disposed.
+func (i *Image) IsDisposed() bool {
+	return i.texture == nil
+}
+
 // ReplacePixels replaces the pixels of the image with p.
 //
 // The given p must represent RGBA pre-multiplied alpha values. len(p) must equal to 4 * (image width) * (image height).