Possible leak due to thread safety issue

[Chris Coulson]

CompositorFrameHandle isn't thread-safe and is intended to be used on the UI thread (although it's also used on the QML render thread when the UI thread is paused). As a consequence it doesn't use a thread-safe reference count.

When calling Compositor::DidSwapCompositorFrame(), we check that the returned CompositorFrameHandles don't have a reference outside of the compositor, before handing them off to the compositor thread where they are deleted. However, even when there are no references held outside of the compositor, there is a race when the UI and compositor threads release their reference that means we may leak the object.

This is fixed on trunk by http://bazaar.launchpad.net/~oxide-developers/oxide/oxide.trunk/revision/892