Currently, each frame callback sent by KWin has the current time in
nanoseconds, but the protocol spec states that we have to send the time
Also, it's worth to mention that m_timeSinceStart isn't actually "time
since start," it's rather accumulated duration of all painting cycles.
If there is something to draw and it takes quite a while to compose the
scene, maybe m_timeSinceStart will be close enough to the current time.