fix: adapt to new function signatures

To allow to bump to latest kovidgoyal/imaging release

services/thumbnails/pkg/thumbnail/processor.go

@@ -10,20 +10,20 @@ import (
 // Processor processes the thumbnail by applying different transformations to it.
 type Processor interface {
 	ID() string
-	Process(img image.Image, width, height int, filter imaging.ResampleFilter) *image.NRGBA
+	Process(img image.Image, width, height int, filter imaging.ResampleFilter) image.Image
 }
 
 // DefinableProcessor is the simplest processor, it holds a replaceable image converter function.
 type DefinableProcessor struct {
 	Slug      string
-	Converter func(img image.Image, width, height int, filter imaging.ResampleFilter) *image.NRGBA
+	Converter func(img image.Image, width, height int, filter imaging.ResampleFilter) image.Image
 }
 
 // ID returns the processor identification.
 func (p DefinableProcessor) ID() string { return p.Slug }
 
 // Process transforms the given image.
-func (p DefinableProcessor) Process(img image.Image, width, height int, filter imaging.ResampleFilter) *image.NRGBA {
+func (p DefinableProcessor) Process(img image.Image, width, height int, filter imaging.ResampleFilter) image.Image {
 	return p.Converter(img, width, height, filter)
 }
 
@@ -35,7 +35,7 @@ func ProcessorFor(id, fileType string) (DefinableProcessor, error) {
 	case "resize":
 		return DefinableProcessor{Slug: strings.ToLower(id), Converter: imaging.Resize}, nil
 	case "fill":
-		return DefinableProcessor{Slug: strings.ToLower(id), Converter: func(img image.Image, width, height int, filter imaging.ResampleFilter) *image.NRGBA {
+		return DefinableProcessor{Slug: strings.ToLower(id), Converter: func(img image.Image, width, height int, filter imaging.ResampleFilter) image.Image {
 			return imaging.Fill(img, width, height, imaging.Center, filter)
 		}}, nil
 	case "thumbnail":