app: fix #6961 Scaled image bug

With large image sizes a 32-bit int is not enough for the intermediate
computations, which byte per pixel, width and height are.

So, just like the function below it does: gimp_gegl_pyramid_get_memsize,
we will cast these to gint64.

Thanks to Massimo Valentini for finding the cause.

app/core/gimp-memsize.c

@@ -285,9 +285,9 @@ gimp_gegl_buffer_get_memsize (GeglBuffer *buffer)
     {
       const Babl *format = gegl_buffer_get_format (buffer);
 
-      return (babl_format_get_bytes_per_pixel (format) *
-              gegl_buffer_get_width (buffer) *
-              gegl_buffer_get_height (buffer) +
+      return ((gint64) babl_format_get_bytes_per_pixel (format) *
+              (gint64) gegl_buffer_get_width (buffer) *
+              (gint64) gegl_buffer_get_height (buffer) +
               gimp_g_object_get_memsize (G_OBJECT (buffer)));
     }