Better builtin avatar generator (#17707)

This PR fixes the builtin avatar generator.

1. The random background color makes some images very dirty. So now we only use white background for avatars.
2. We use left-right mirror avatars to satisfy #14799
3. Fix a small padding error in the algorithm

modules/avatar/identicon/identicon_test.go

@@ -0,0 +1,42 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+//go:build test_avatar_identicon
+//  +build test_avatar_identicon
+
+package identicon
+
+import (
+	"image/color"
+	"image/png"
+	"os"
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenerate(t *testing.T) {
+	dir, _ := os.Getwd()
+	dir = dir + "/testdata"
+	if st, err := os.Stat(dir); err != nil || !st.IsDir() {
+		t.Errorf("can not save generated images to %s", dir)
+	}
+
+	backColor := color.White
+	imgMaker, err := New(64, backColor, DarkColors...)
+	assert.NoError(t, err)
+	for i := 0; i < 100; i++ {
+		s := strconv.Itoa(i)
+		img := imgMaker.Make([]byte(s))
+
+		f, err := os.Create(dir + "/" + s + ".png")
+		if !assert.NoError(t, err) {
+			continue
+		}
+		defer f.Close()
+		err = png.Encode(f, img)
+		assert.NoError(t, err)
+	}
+}