Fixes a condition where too much data on the local clipboard causes the client to...

Fixes a condition where too much data on the local clipboard causes the client to exceed the max heap size and exit.  Since the server will, by default, discard clipboard transfers greater than 256KB anyway, a parameter was added which sets the default max clipboard size that the client will hold to 256KB also.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@5138 3789f03b-4d11-0410-bbf8-ca57d06f2519