Fix GeoGebra thumbnails when libvips support is enabled

For GeoGebra files we where still using the imaging package to decode the
embedded png, but handed it of to vips for scaling, which can't work. Now
we use vips for decoding the image as well.

services/thumbnails/pkg/errors/error.go

@@ -11,6 +11,8 @@ var (
 	ErrNoEncoderForType = errors.New("thumbnails: no encoder for this type found")
 	// ErrNoImageFromAudioFile defines an error when an image cannot be extracted from an audio file
 	ErrNoImageFromAudioFile = errors.New("thumbnails: could not extract image from audio file")
+	// ErrNoConverterForExtractedImageFromGgsFile defines an error when the extracted image from an ggs file could not be converted
+	ErrNoConverterForExtractedImageFromGgsFile = errors.New("thumbnails: could not find converter for image extracted from ggs file")
 	// ErrNoConverterForExtractedImageFromAudioFile defines an error when the extracted image from an audio file could not be converted
 	ErrNoConverterForExtractedImageFromAudioFile = errors.New("thumbnails: could not find converter for image extracted from audio file")
 	// ErrCS3AuthorizationMissing defines an error when the CS3 authorization is missing

services/thumbnails/pkg/preprocessor/preprocessor.go

@@ -12,7 +12,6 @@ import (
 	"mime"
 	"strings"
 
-	"github.com/kovidgoyal/imaging"
 	"github.com/pkg/errors"
 	"golang.org/x/image/font"
 	"golang.org/x/image/font/opentype"
@@ -60,8 +59,11 @@ func (g GgsDecoder) Convert(r io.Reader) (interface{}, error) {
 			if err != nil {
 				return nil, err
 			}
-
-			img, err := imaging.Decode(thumbnail, imaging.AutoOrientation(true))
+			converter := ForType("image/png", nil)
+			if converter == nil {
+				return nil, thumbnailerErrors.ErrNoConverterForExtractedImageFromGgsFile
+			}
+			img, err := converter.Convert(thumbnail)
 			if err != nil {
 				return nil, errors.Wrap(err, `could not decode the image`)
 			}