Fix the way the dpr processing option affects offsets and paddings

processing/pipeline.go

@@ -29,6 +29,8 @@ type pipelineContext struct {
 	wscale float64
 	hscale float64
 
+	dprScale float64
+
 	iccImported bool
 }
 
@@ -59,5 +61,7 @@ func (p pipeline) Run(ctx context.Context, img *vips.Image, po *options.Processi
 		}
 	}
 
+	img.SetDouble("imgproxy-dpr-scale", pctx.dprScale)
+
 	return nil
 }