Flatten image with alpha when saving to jpeg; Make white default background

process.go

@@ -336,7 +336,7 @@ func transformImage(ctx context.Context, img **C.struct__VipsImage, data []byte,
 		checkTimeout(ctx)
 	}
 
-	if hasAlpha && po.Flatten {
+	if hasAlpha && (po.Flatten || po.Format == imageTypeJPEG) {
 		if err = vipsFlatten(img, po.Background); err != nil {
 			return err
 		}

processing_options.go

@@ -702,6 +702,7 @@ func defaultProcessingOptions(headers *processingHeaders) (*processingOptions, e
 		Enlarge:     false,
 		Quality:     conf.Quality,
 		Format:      imageTypeUnknown,
+		Background:  color{255, 255, 255},
 		Blur:        0,
 		Sharpen:     0,
 		Watermark:   watermarkOptions{Opacity: 1, Replicate: false, Gravity: gravityCenter},