pango: Add a property to compensate for display response time

When measuring video latency, one mechanism involves taking a photo with a camera of two screens showing the test video overlayed with timeoverlay or clockoverlay. In these cases, if the display's pixel response time is crappy, you will see ghosting due to which it can be quite difficult to discern what the current timestamp being shown is.

This commit adds a property that also shows the timestamp in a different (sequentially predictable) location every frame, which makes it easy to tell what the latest rendered timestamp is.

For bonus points, you can also use the fade-time of the previous frame to measure with sub-framerate accuracy when the photo was taken, not just clamped to the framerate, giving you a higher precision latency value.