ebiten: Remove the returning value from (*Image).Dispose

Updates #1380

image.go

@@ -678,20 +678,17 @@ func (i *Image) Set(x, y int, clr color.Color) {
 // However, calling Dispose explicitly is helpful if memory usage matters.
 //
 // When the image is disposed, Dipose does nothing.
-//
+func (i *Image) Dispose() {
-// Dipose always return nil as of 1.5.0.
-func (i *Image) Dispose() error {
 	i.copyCheck()
 
 	if i.isDisposed() {
-		return nil
+		return
 	}
 	if i.isSubImage() {
-		return nil
+		return
 	}
 	i.mipmap.MarkDisposed()
 	i.mipmap = nil
-	return nil
 }
 
 // ReplacePixels replaces the pixels of the image with p.

image_test.go

@@ -383,9 +383,7 @@ func TestImageDispose(t *testing.T) {
 		return
 	}
 	img.Fill(color.White)
-	if err := img.Dispose(); err != nil {
+	img.Dispose()
-		t.Errorf("img.Dipose() returns error: %v", err)
-	}
 
 	// The color is transparent (color.RGBA{}).
 	// Note that the value's type must be color.RGBA.

uicontext.go

@@ -81,13 +81,13 @@ func (c *uiContext) updateOffscreen() {
 	c.outsideSizeUpdated = false
 
 	if c.screen != nil {
-		_ = c.screen.Dispose()
+		c.screen.Dispose()
 		c.screen = nil
 	}
 
 	if c.offscreen != nil {
 		if w, h := c.offscreen.Size(); w != sw || h != sh {
-			_ = c.offscreen.Dispose()
+			c.offscreen.Dispose()
 			c.offscreen = nil
 		}
 	}