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/avatar.go

@@ -8,16 +8,15 @@ import (
 	"bytes"
 	"fmt"
 	"image"
-	"image/color/palette"
+	"image/color"
 
 	_ "image/gif"  // for processing gif images
 	_ "image/jpeg" // for processing jpeg images
 	_ "image/png"  // for processing png images
 
+	"code.gitea.io/gitea/modules/avatar/identicon"
 	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/util"
 
-	"github.com/issue9/identicon"
 	"github.com/nfnt/resize"
 	"github.com/oliamb/cutter"
 )
@@ -28,20 +27,8 @@ const AvatarSize = 290
 // RandomImageSize generates and returns a random avatar image unique to input data
 // in custom size (height and width).
 func RandomImageSize(size int, data []byte) (image.Image, error) {
-	randExtent := len(palette.WebSafe) - 32
-	integer, err := util.RandomInt(int64(randExtent))
-	if err != nil {
-		return nil, fmt.Errorf("util.RandomInt: %v", err)
-	}
-	colorIndex := int(integer)
-	backColorIndex := colorIndex - 1
-	if backColorIndex < 0 {
-		backColorIndex = randExtent - 1
-	}
-
-	// Define size, background, and forecolor
-	imgMaker, err := identicon.New(size,
-		palette.WebSafe[backColorIndex], palette.WebSafe[colorIndex:colorIndex+32]...)
+	// we use white as background, and use dark colors to draw blocks
+	imgMaker, err := identicon.New(size, color.White, identicon.DarkColors...)
 	if err != nil {
 		return nil, fmt.Errorf("identicon.New: %v", err)
 	}