[gimp] app: fix selection mask offset in do_layer_blend()

[Michael Natterer <mitch src gnome org>]

commit db386eb361915cf00c7626ad7203e528cfb5f671
Author: Michael Natterer <mitch gimp org>
Date:   Sat Jul 4 13:28:59 2015 +0200

    app: fix selection mask offset in do_layer_blend()
    
    Fixes painting when there is both a layer offset and a selection.
    Offsets between offsets are hard...

 app/paint/gimppaintcore-loops.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/app/paint/gimppaintcore-loops.c b/app/paint/gimppaintcore-loops.c
index 5be3474..fb9df17 100644
--- a/app/paint/gimppaintcore-loops.c
+++ b/app/paint/gimppaintcore-loops.c
@@ -308,8 +308,8 @@ do_layer_blend (GeglBuffer  *src_buffer,
   roi.width  = gimp_temp_buf_get_width (paint_buf);
   roi.height = gimp_temp_buf_get_height (paint_buf);
 
-  mask_roi.x = roi.x + mask_x_offset;
-  mask_roi.y = roi.y + mask_y_offset;
+  mask_roi.x = roi.x - mask_x_offset;
+  mask_roi.y = roi.y - mask_y_offset;
   mask_roi.width  = roi.width;
   mask_roi.height = roi.height;