Improve avatar uploading / resizing / compressing, remove Fomantic card module (#24653)

Fixes: #8972
Fixes: #24263

And I think it also (partially) fix #24263 (no need to convert) ,
because users could upload any supported image format if it isn't larger
than AVATAR_MAX_ORIGIN_SIZE


The main idea: 

* if the uploaded file size is not larger than AVATAR_MAX_ORIGIN_SIZE,
use the origin
* if the resized size is larger than the origin, use the origin

Screenshots:

JPG:

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/70e98bb0-ecb9-4c4e-a89f-4a37d4e37f8e)

</details>

APNG:

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/9055135b-5e2d-4152-bd72-596fcb7c6671)


![image](https://github.com/go-gitea/gitea/assets/2114189/50364caf-f7f6-4241-a289-e485fe4cd582)

</details>

WebP (animated)

<details>


![image](https://github.com/go-gitea/gitea/assets/2114189/f642eb85-498a-49a5-86bf-0a7b04089ae0)

</details>

The only exception: if a WebP image is larger than MaxOriginSize and it
is animated, then current `webp` package can't decode it, so only in
this case it isn't supported. IMO no need to support such case: why a
user would upload a 1MB animated webp as avatar? crazy .....

---------

Co-authored-by: silverwind <me@silverwind.io>

modules/avatar/avatar.go

@@ -5,13 +5,14 @@ package avatar
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"image"
 	"image/color"
+	"image/png"
 
 	_ "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"
@@ -22,8 +23,11 @@ import (
 	_ "golang.org/x/image/webp" // for processing webp images
 )
 
-// AvatarSize returns avatar's size
-const AvatarSize = 290
+// DefaultAvatarSize is the target CSS pixel size for avatar generation. It is
+// multiplied by setting.Avatar.RenderedSizeFactor and the resulting size is the
+// usual size of avatar image saved on server, unless the original file is smaller
+// than the size after resizing.
+const DefaultAvatarSize = 256
 
 // RandomImageSize generates and returns a random avatar image unique to input data
 // in custom size (height and width).
@@ -39,28 +43,44 @@ func RandomImageSize(size int, data []byte) (image.Image, error) {
 // RandomImage generates and returns a random avatar image unique to input data
 // in default size (height and width).
 func RandomImage(data []byte) (image.Image, error) {
-	return RandomImageSize(AvatarSize, data)
+	return RandomImageSize(DefaultAvatarSize*setting.Avatar.RenderedSizeFactor, data)
 }
 
-// Prepare accepts a byte slice as input, validates it contains an image of an
-// acceptable format, and crops and resizes it appropriately.
-func Prepare(data []byte) (*image.Image, error) {
-	imgCfg, _, err := image.DecodeConfig(bytes.NewReader(data))
+// processAvatarImage process the avatar image data, crop and resize it if necessary.
+// the returned data could be the original image if no processing is needed.
+func processAvatarImage(data []byte, maxOriginSize int64) ([]byte, error) {
+	imgCfg, imgType, err := image.DecodeConfig(bytes.NewReader(data))
 	if err != nil {
-		return nil, fmt.Errorf("DecodeConfig: %w", err)
+		return nil, fmt.Errorf("image.DecodeConfig: %w", err)
 	}
+
+	// for safety, only accept known types explicitly
+	if imgType != "png" && imgType != "jpeg" && imgType != "gif" && imgType != "webp" {
+		return nil, errors.New("unsupported avatar image type")
+	}
+
+	// do not process image which is too large, it would consume too much memory
 	if imgCfg.Width > setting.Avatar.MaxWidth {
-		return nil, fmt.Errorf("Image width is too large: %d > %d", imgCfg.Width, setting.Avatar.MaxWidth)
+		return nil, fmt.Errorf("image width is too large: %d > %d", imgCfg.Width, setting.Avatar.MaxWidth)
 	}
 	if imgCfg.Height > setting.Avatar.MaxHeight {
-		return nil, fmt.Errorf("Image height is too large: %d > %d", imgCfg.Height, setting.Avatar.MaxHeight)
+		return nil, fmt.Errorf("image height is too large: %d > %d", imgCfg.Height, setting.Avatar.MaxHeight)
+	}
+
+	// If the origin is small enough, just use it, then APNG could be supported,
+	// otherwise, if the image is processed later, APNG loses animation.
+	// And one more thing, webp is not fully supported, for animated webp, image.DecodeConfig works but Decode fails.
+	// So for animated webp, if the uploaded file is smaller than maxOriginSize, it will be used, if it's larger, there will be an error.
+	if len(data) < int(maxOriginSize) {
+		return data, nil
 	}
 
 	img, _, err := image.Decode(bytes.NewReader(data))
 	if err != nil {
-		return nil, fmt.Errorf("Decode: %w", err)
+		return nil, fmt.Errorf("image.Decode: %w", err)
 	}
 
+	// try to crop and resize the origin image if necessary
 	if imgCfg.Width != imgCfg.Height {
 		var newSize, ax, ay int
 		if imgCfg.Width > imgCfg.Height {
@@ -74,13 +94,33 @@ func Prepare(data []byte) (*image.Image, error) {
 		img, err = cutter.Crop(img, cutter.Config{
 			Width:  newSize,
 			Height: newSize,
-			Anchor: image.Point{ax, ay},
+			Anchor: image.Point{X: ax, Y: ay},
 		})
 		if err != nil {
 			return nil, err
 		}
 	}
 
-	img = resize.Resize(AvatarSize, AvatarSize, img, resize.Bilinear)
-	return &img, nil
+	targetSize := uint(DefaultAvatarSize * setting.Avatar.RenderedSizeFactor)
+	img = resize.Resize(targetSize, targetSize, img, resize.Bilinear)
+
+	// try to encode the cropped/resized image to png
+	bs := bytes.Buffer{}
+	if err = png.Encode(&bs, img); err != nil {
+		return nil, err
+	}
+	resized := bs.Bytes()
+
+	// usually the png compression is not good enough, use the original image (no cropping/resizing) if the origin is smaller
+	if len(data) <= len(resized) {
+		return data, nil
+	}
+
+	return resized, nil
+}
+
+// ProcessAvatarImage process the avatar image data, crop and resize it if necessary.
+// the returned data could be the original image if no processing is needed.
+func ProcessAvatarImage(data []byte) ([]byte, error) {
+	return processAvatarImage(data, setting.Avatar.MaxOriginSize)
 }