ui/vnc: Only report/use TIGHT_PNG encoding if enabled.

ui/vnc: Only report/use TIGHT_PNG encoding if enabled.

If TIGHT_PNG is not enabled by the --enable-vnc-png configure flag
then do not report to the client that it is supported.

Also, since TIGHT_PNG is the same as the TIGHT encoding but with the
filter/copy replaced with PNG data, adding it to the supported
encodings list when it is disabled will cause the TIGHT encoding to be
used even though the client requested TIGHT_PNG.

Signed-off-by: Joel Martin <github@martintribe.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>