Fix png_set_option to work.

png.c

diff --git a/png.c b/png.c
index 88cf9e7..cc6029e 100644
--- a/png.c
+++ b/png.c
@@ -4344,7 +4344,7 @@
       png_uint_32 setting = (2U + (onoff != 0)) << option;
       png_uint_32 current = png_ptr->options;
 
-      png_ptr->options = (png_uint_32)(((current & ~mask) | setting) & 0xff);
+      png_ptr->options = (png_uint_32)((current & ~mask) | setting);
 
       return (int)(current & mask) >> option;
    }