This brings unnecessary upstream incompatibility and API change
(it changes priority of different flush types in some cases).
The original reason for this patch was rhbz#844791, which has been
later worked-around in tigervnc anyways (b5822f32ab5043b4e).
Zlib upstream fixed the original issue by commit
c661c374e8df43365738daa09f4390d61bc57f08, which is for some time
available in Fedora's zlib (220.127.116.11+).