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/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`)
 			}