Check if there is an alpha channel before flattening

vips.go

@@ -274,13 +274,14 @@ func vipsFlattenBackground(image *C.VipsImage, background Color) (*C.VipsImage,
 		C.double(background.B),
 	}
 
+	if vipsHasAlpha(image) {
 		err := C.vips_flatten_background_brigde(image, &outImage, (*C.double)(&backgroundC[0]))
 		if int(err) != 0 {
 			return nil, catchVipsError()
 		}
-
 	C.g_object_unref(C.gpointer(image))
 	image = outImage
+	}
 
 	return image, nil
 }