Update libpng to 1.2.33.

third_party/libpng/pngwtran.c

Index: third_party/libpng/pngwtran.c
===================================================================
--- third_party/libpng/pngwtran.c	(revision 7204)
+++ third_party/libpng/pngwtran.c	(working copy)
@@ -25,7 +25,7 @@
 
 #if defined(PNG_WRITE_USER_TRANSFORM_SUPPORTED)
    if (png_ptr->transformations & PNG_USER_TRANSFORM)
-      if(png_ptr->write_user_transform_fn != NULL)
+      if (png_ptr->write_user_transform_fn != NULL)
         (*(png_ptr->write_user_transform_fn)) /* user write transform function */
           (png_ptr,                    /* png_ptr */
            &(png_ptr->row_info),       /* row_info:     */
@@ -558,8 +558,8 @@
             png_uint_32 s0   = (*(rp  ) << 8) | *(rp+1);
             png_uint_32 s1   = (*(rp+2) << 8) | *(rp+3);
             png_uint_32 s2   = (*(rp+4) << 8) | *(rp+5);
-            png_uint_32 red  = (png_uint_32)((s0-s1) & 0xffffL);
-            png_uint_32 blue = (png_uint_32)((s2-s1) & 0xffffL);
+            png_uint_32 red  = (png_uint_32)((s0 - s1) & 0xffffL);
+            png_uint_32 blue = (png_uint_32)((s2 - s1) & 0xffffL);
             *(rp  ) = (png_byte)((red >> 8) & 0xff);
             *(rp+1) = (png_byte)(red & 0xff);
             *(rp+4) = (png_byte)((blue >> 8) & 0xff);