[gimp] app: in GimpTileHandlerValidate, clear data when allocating tile

[Ell <ell src gnome org>]

commit e1e4ba9c8b18c860ceed0cedf21a793517da3d06
Author: Ell <ell_se yahoo com>
Date:   Fri Aug 3 11:58:01 2018 -0400

    app: in GimpTileHandlerValidate, clear data when allocating tile
    
    In GimpTileHandlerValidate, when allocating a new tile upon a
    TILE_GET command, but not rendering the whole tile, clear the tile
    data before rendering, so that the unrendered regions of the tile
    contain zeros, rather than junk.

 app/gegl/gimptilehandlervalidate.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)
---
diff --git a/app/gegl/gimptilehandlervalidate.c b/app/gegl/gimptilehandlervalidate.c
index e9b6d953a1..eb0debd712 100644
--- a/app/gegl/gimptilehandlervalidate.c
+++ b/app/gegl/gimptilehandlervalidate.c
@@ -260,15 +260,20 @@ gimp_tile_handler_validate_validate (GeglTileSource *source,
           gint n_rects;
           gint i;
 
-          if (! tile)
-            tile = gegl_tile_handler_create_tile (GEGL_TILE_HANDLER (source),
-                                                  x, y, 0);
-
           cairo_region_subtract_rectangle (validate->dirty_region, &tile_rect);
 
           tile_bpp    = babl_format_get_bytes_per_pixel (validate->format);
           tile_stride = tile_bpp * validate->tile_width;
 
+          if (! tile)
+            {
+              tile = gegl_tile_handler_create_tile (GEGL_TILE_HANDLER (source),
+                                                    x, y, 0);
+
+              memset (gegl_tile_get_data (tile),
+                      0, tile_stride * validate->tile_height);
+            }
+
           gegl_tile_lock (tile);
 
           n_rects = cairo_region_num_rectangles (tile_region);