Disable png filters for lower compression levels. This should lower
the CPU consumption and reduce encoding time.

This isn't in tight_conf because:
* tight_conf structure must not change, because it's shared with other
  tight implementations (libvncserver, etc..).
* it'd exceed the 80 col limit.
* PNG_ macros are only defined if CONFIG_VNC_PNG is defined
Signed-off-by: NCorentin Chary <corentincj@iksaif.net>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>