refactor(thumbnails): separate out 'kovidgoyal/imaging' dependencies

Move code using the 'kovidgoyal/imaging' package to separate files
to make it easier to create alternative implementations based on build
tags.

services/thumbnails/pkg/thumbnail/encoding_imaging.go

@@ -0,0 +1,54 @@
+package thumbnail
+
+import (
+	"image"
+	"image/jpeg"
+	"image/png"
+	"io"
+
+	"github.com/owncloud/ocis/v2/services/thumbnails/pkg/errors"
+)
+
+// PngEncoder encodes to png
+type PngEncoder struct{}
+
+// Encode encodes to png format
+func (e PngEncoder) Encode(w io.Writer, img interface{}) error {
+	m, ok := img.(image.Image)
+	if !ok {
+		return errors.ErrInvalidType
+	}
+	return png.Encode(w, m)
+}
+
+// Types returns the png suffix
+func (e PngEncoder) Types() []string {
+	return []string{typePng}
+}
+
+// MimeType returns the mimetype for png files.
+func (e PngEncoder) MimeType() string {
+	return "image/png"
+}
+
+// JpegEncoder encodes to jpg
+type JpegEncoder struct{}
+
+// Encode encodes to jpg
+func (e JpegEncoder) Encode(w io.Writer, img interface{}) error {
+	m, ok := img.(image.Image)
+	if !ok {
+		return errors.ErrInvalidType
+	}
+	return jpeg.Encode(w, m, nil)
+}
+
+// Types returns the jpg suffixes.
+func (e JpegEncoder) Types() []string {
+	return []string{typeJpeg, typeJpg}
+}
+
+// MimeType returns the mimetype for jpg files.
+func (e JpegEncoder) MimeType() string {
+	return "image/jpeg"
+}