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
in milliseconds.
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.