Turns out that TXImage wasn't just being overly cautious with the custom error

Turns out that TXImage wasn't just being overly cautious with the custom error
handler; the shared memory extension is actually this broken so it will return
success all the way through even when you're on a remote connection where
things cannot possibly work.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4505 3789f03b-4d11-0410-bbf8-ca57d06f2519