Keep frame clock running if waiting for frame tick

Keep frame clock running if waiting for frame tick

If there is something interested in synchronizing to a frame tick, then
keep the frame clock running, even if there are no updates.

This is need mainly when something starts rendering, but also when
something renders much slower than the frame clock (so it is essentially
constantly "starting"). Such an application will not draw anything until
it gets a new frame tick, which it won't get as the frame clock is
waiting for something to start drawing.