[devel] Bump to version libpng-1.5.0beta13

pngtest.c

@@ -1621,4 +1621,4 @@ main(int argc, char *argv[])
 }
 
 /* Generate a compiler error if there is an old png.h in the search path. */
-typedef version_1_5_0beta12 your_png_h_is_not_version_1_5_0beta12;
+typedef version_1_5_0beta13 your_png_h_is_not_version_1_5_0beta13;