[gimp] Issue #5208 - paint brush is broken when aspect ratio is set to negative

[Ell <ell src gnome org>]

commit c4a201eaf4e7cf6ec8a76768e6bb27dcbe2e94f8
Author: Ell <ell_se yahoo com>
Date:   Fri Jun 12 17:20:23 2020 +0300

    Issue #5208 - paint brush is broken when aspect ratio is set to negative
    
    Fix horizontal downscaling of brush mipmap levels with odd width.
    We'd previously fail to skip the last pixel of each input row,
    which isn't included in the output when the width is odd, causing
    subsequent output rows to be shifted to the right.

 app/core/gimpbrush-mipmap.cc | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)
---
diff --git a/app/core/gimpbrush-mipmap.cc b/app/core/gimpbrush-mipmap.cc
index ae394dc3e9..7c3b061130 100644
--- a/app/core/gimpbrush-mipmap.cc
+++ b/app/core/gimpbrush-mipmap.cc
@@ -292,16 +292,20 @@ struct MipmapAlgorithms
       [=] (gint offset,
            gint size)
       {
-        const T *src  = (const T *) gimp_temp_buf_get_data (source);
-        T       *dest = (T       *) gimp_temp_buf_get_data (destination);
+        const T *src0        = (const T *) gimp_temp_buf_get_data (source);
+        T       *dest0       = (T       *) gimp_temp_buf_get_data (destination);
+        gint     src_stride  = N * gimp_temp_buf_get_width (source);
+        gint     dest_stride = N * gimp_temp_buf_get_width (destination);
         gint     y;
 
-        src  += offset * gimp_temp_buf_get_width (source)      * N;
-        dest += offset * gimp_temp_buf_get_width (destination) * N;
+        src0  += offset * src_stride;
+        dest0 += offset * dest_stride;
 
         for (y = 0; y < size; y++)
           {
-            gint x;
+            const T *src  = src0;
+            T       *dest = dest0;
+            gint     x;
 
             for (x = 0; x < width; x++)
               {
@@ -313,6 +317,9 @@ struct MipmapAlgorithms
                 src  += 2 * N;
                 dest += N;
               }
+
+            src0  += src_stride;
+            dest0 += dest_stride;
           }
       });