Do not touch the alpha layer in pipi_contrast().

git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libpipi/trunk@3597 92316355-f0b4-4df1-b90c-862c8a59935f

pipi/filter/color.c

@@ -155,7 +155,7 @@ pipi_image_t *pipi_contrast(pipi_image_t *src, double val)
                     dstdata[d + 1] = p < 0. ? 0. : p > 1. ? 1. : p;
                     p = (srcdata[d + 2] - 0.5) * val + 0.5;
                     dstdata[d + 2] = p < 0. ? 0. : p > 1. ? 1. : p;
-                    p = (srcdata[d + 3] - 0.5) * val + 0.5;
+                    p = srcdata[d + 3];
                     dstdata[d + 3] = p;
                 }
             }