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/preprocessor/preprocessor.go

@@ -27,18 +27,6 @@ type FileConverter interface {
 	Convert(r io.Reader) (interface{}, error)
 }
 
-// ImageDecoder is a converter for the image file
-type ImageDecoder struct{}
-
-// Convert reads the image file and returns the thumbnail image
-func (i ImageDecoder) Convert(r io.Reader) (interface{}, error) {
-	img, err := imaging.Decode(r, imaging.AutoOrientation(true))
-	if err != nil {
-		return nil, errors.Wrap(err, `could not decode the image`)
-	}
-	return img, nil
-}
-
 // GifDecoder is a converter for the gif file
 type GifDecoder struct{}
 

services/thumbnails/pkg/preprocessor/preprocessor_imaging.go

@@ -0,0 +1,20 @@
+package preprocessor
+
+import (
+	"io"
+
+	"github.com/kovidgoyal/imaging"
+	"github.com/pkg/errors"
+)
+
+// ImageDecoder is a converter for the image file
+type ImageDecoder struct{}
+
+// Convert reads the image file and returns the thumbnail image
+func (i ImageDecoder) Convert(r io.Reader) (interface{}, error) {
+	img, err := imaging.Decode(r, imaging.AutoOrientation(true))
+	if err != nil {
+		return nil, errors.Wrap(err, `could not decode the image`)
+	}
+	return img, nil
+}