Log Message:
tigervnc: update to 1.15.0.1.20.13.

pkgsrc change:
Switch to fltk13, this release does not accept fltk 1.4.

Changes:
* The native viewer and Unix servers now support the back and
  forward mouse buttons
* The Unix servers can now handle any number of keys that are
  missing from the server keyboard layout, rather than just a few
* x0vncserver now supports clipboard redirection
* The native viewer can remember the username and password when
  reconnecting
* Both the native and Java viewer can now show a standard arrow
  cursor, not just a dot cursor, if the server cursor is blank
* vncpasswd can now check password complexity with pwquality