Tight PNG VNC encoding is sent even when --disable-vnc-png is set

This bug exists in 0.14.1 and also in 9312805d33e8b (Jun 17, 2011) in the master git repo.

The "Tight PNG" encoding is a derivative of the "Tight" encoding that replaces zlib encoded rects with PNG encoded data instead. However, when the "Tight PNG" encoding is disabled (--disable-vnc-png), the server will send frame buffer updates that are marked as "Tight PNG" but in fact contain zlib encoded regions (i.e. it's really "tight" protocol).

The "Tight PNG" encoding should only be used when --enable-vnc-png is set.