Compositor: Background Fit UI Scale

Make the fit background relative to the UI scale.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D5150

source/blender/editors/space_node/node_view.c

@@ -347,7 +347,7 @@ static int backimage_fit_exec(bContext *C, wmOperator *UNUSED(op))
 
   BKE_image_release_ibuf(ima, ibuf, lock);
 
-  snode->zoom *= min_ff(facx, facy);
+  snode->zoom *= min_ff(facx, facy) * U.dpi_fac;
 
   snode->xof = 0;
   snode->yof = 0;