autocontrast: fix alpha handling.

git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libpipi/trunk@4879 92316355-f0b4-4df1-b90c-862c8a59935f

pipi/filter/autocontrast.c

@@ -100,6 +100,8 @@ pipi_image_t *pipi_autocontrast(pipi_image_t *src)
                     = (srcdata[4 * (y * w + x) + 1] - min) * t;
                 dstdata[4 * (y * w + x) + 2]
                     = (srcdata[4 * (y * w + x) + 2] - min) * t;
+                dstdata[4 * (y * w + x) + 3]
+                    = srcdata[4 * (y * w + x) + 3];
             }
         }
     }