hasNewFrame can return NO from a new frame handler

[bangnoise]

Sometimes the SyphonClientBase class calls the frameHandler, but the boolean "hasNewFrame" indicates there is no new frame to draw (This behaviour is logged in our client demo app) which is inconsistant

This is likely because we rely on a glFlush (the sync point for the IOSurface) after unbindAndPublish. This does mean the remote client can have the new-frame handler invoked prior to the IOSurface update being synced. In practice, it is likely (but not certain) that sync will have happened before the client actually uses the surface.

Adding this certainty would require performing the flush ourselves in unbindAndPublish - after unbinding and before publishing.