[lipstick] No throttling when compositor is hidden

 - For power consumption, clients should not post frames
   when compositor is hidden.

 - But, for reliability, compositor should not block clients
   even though it is hidden.

 => This commit touches the second issue. Another fix should
    be done for announcing display off for clients.
Merge pull request #86 from mikkoharju/displayoff

[lipstick] No throttling when compositor is hidden