[MM-29241] Image logic refactor (#17702)

* Image logic refactor

* Add missing translations

* Improve prepareImage

* Use iota

* Limit image encoder concurrency

* Unexport validation methods

* Avoid shortening on exported names

* Remove unnecessary complexity

app/imaging/encode_test.go

@@ -0,0 +1,90 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package imaging
+
+import (
+	"bytes"
+	"image"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestNewEncoder(t *testing.T) {
+	t.Run("invalid options", func(t *testing.T) {
+		e, err := NewEncoder(EncoderOptions{
+			ConcurrencyLevel: -1,
+		})
+		require.Nil(t, e)
+		require.Error(t, err)
+	})
+
+	t.Run("empty options", func(t *testing.T) {
+		e, err := NewEncoder(EncoderOptions{})
+		require.NotNil(t, e)
+		require.NoError(t, err)
+		require.Nil(t, e.sem)
+	})
+
+	t.Run("valid options", func(t *testing.T) {
+		e, err := NewEncoder(EncoderOptions{
+			ConcurrencyLevel: 4,
+		})
+		require.NotNil(t, e)
+		require.NoError(t, err)
+		require.NotNil(t, e.sem)
+		require.Equal(t, 4, cap(e.sem))
+	})
+}
+
+func TestEncoderEncode(t *testing.T) {
+	t.Run("default options", func(t *testing.T) {
+		e, err := NewEncoder(EncoderOptions{})
+		require.NotNil(t, e)
+		require.NoError(t, err)
+
+		var buf bytes.Buffer
+		rawImg := image.NewRGBA(image.Rect(0, 0, 1280, 1024))
+		err = e.EncodePNG(&buf, rawImg)
+		require.NoError(t, err)
+		require.NotEmpty(t, buf)
+
+		err = e.EncodeJPEG(&buf, rawImg, 50)
+		require.NoError(t, err)
+		require.NotEmpty(t, buf)
+	})
+
+	t.Run("concurrency bounded", func(t *testing.T) {
+		e, err := NewEncoder(EncoderOptions{
+			ConcurrencyLevel: 1,
+		})
+		require.NotNil(t, e)
+		require.NoError(t, err)
+
+		rawImg := image.NewRGBA(image.Rect(0, 0, 1280, 1024))
+
+		var wg sync.WaitGroup
+		wg.Add(2)
+
+		go func() {
+			defer wg.Done()
+			var buf bytes.Buffer
+			err := e.EncodePNG(&buf, rawImg)
+			require.NoError(t, err)
+			require.NotEmpty(t, buf)
+		}()
+
+		go func() {
+			defer wg.Done()
+			var buf bytes.Buffer
+			err := e.EncodeJPEG(&buf, rawImg, 50)
+			require.NoError(t, err)
+			require.NotEmpty(t, buf)
+		}()
+
+		wg.Wait()
+		require.Empty(t, e.sem)
+	})
+}