MM-49395: Increase compression for previews and emojis (#21985)

We also reduce the max size of uploaded emojis to 512KiB.

https://mattermost.atlassian.net/browse/MM-49395

```release-note
Max size of uploaded emojis is now reduced to 512KiB
to reduce image download bandwidth.
```

app/emoji.go

@@ -14,7 +14,6 @@ import (
 	"image/draw"
 	"image/gif"
 	_ "image/jpeg"
-	"image/png"
 	"io"
 	"mime/multipart"
 	"net/http"
@@ -31,7 +30,7 @@ import (
 )
 
 const (
-	MaxEmojiFileSize       = 1 << 20 // 1 MB
+	MaxEmojiFileSize       = 1 << 19 // 512 KiB
 	MaxEmojiWidth          = 128
 	MaxEmojiHeight         = 128
 	MaxEmojiOriginalWidth  = 1028
@@ -155,8 +154,8 @@ func (a *App) UploadEmojiImage(c request.CTX, id string, imageData *multipart.Fi
 				return model.NewAppError("uploadEmojiImage", "api.emoji.upload.large_image.decode_error", nil, "", http.StatusBadRequest).Wrap(err)
 			}
 
-			resized_image := resizeEmoji(img, config.Width, config.Height)
-			if err := png.Encode(newbuf, resized_image); err != nil {
+			resizedImg := resizeEmoji(img, config.Width, config.Height)
+			if err := a.ch.imgEncoder.EncodePNG(newbuf, resizedImg); err != nil {
 				return model.NewAppError("uploadEmojiImage", "api.emoji.upload.large_image.encode_error", nil, "", http.StatusBadRequest).Wrap(err)
 			}
 			buf = newbuf

app/imaging/encode.go

@@ -45,7 +45,9 @@ func NewEncoder(opts EncoderOptions) (*Encoder, error) {
 		e.sem = make(chan struct{}, opts.ConcurrencyLevel)
 	}
 	e.opts = opts
-	e.pngEncoder = &png.Encoder{}
+	e.pngEncoder = &png.Encoder{
+		CompressionLevel: png.BestCompression,
+	}
 	return &e, nil
 }
 

app/user_test.go

@@ -121,7 +121,7 @@ func TestAdjustProfileImage(t *testing.T) {
 	assert.True(t, adjusted.Len() > 0)
 	assert.NotEqual(t, testjpg, adjusted)
 
-	// default image should require adjustment
+	// default image should not require adjustment
 	user := th.BasicUser
 	image, err := th.App.GetDefaultProfileImage(user)
 	require.Nil(t, err)

app/users/profile_picture.go

@@ -160,7 +160,10 @@ func createProfileImage(username string, userID string, initialFont string) ([]b
 
 	buf := new(bytes.Buffer)
 
-	if imgErr := png.Encode(buf, dstImg); imgErr != nil {
+	enc := png.Encoder{
+		CompressionLevel: png.BestCompression,
+	}
+	if imgErr := enc.Encode(buf, dstImg); imgErr != nil {
 		return nil, ImageEncodingError
 	}