[devel] Bump version to libpng-1.4.0beta81

pngtest.c

@@ -1619,4 +1619,4 @@ main(int argc, char *argv[])
 }
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef version_1_4_0beta80 your_png_h_is_not_version_1_4_0beta80;
+typedef version_1_4_0beta81 your_png_h_is_not_version_1_4_0beta81;