Time axis limit in video is not fix when using masks

[relleums]

The time axis in the plot.save_video() has no fix limits on the time axis when using a mask. The masked part of the photon-stream is potentially shorter in time than the full photon-stream. However, the masked part becomes stretched to fit the figure. This results in an alternating scale on the time axis which ruins the illusion of putting and removing the background.