end of recording truncated when using continuous playback

[kdarras]

When listening to a recording from the beginning to the end in reading mode, the end of the playback is not aligned with the end of the spectrogram: the last spectrogram window will display, but the cursor will stop somewhere in-between its start and end. In fact, the last spectrogram will apparently have the same magnification as the previous view before the end of the recording, even though the last view would apply to a shorter part of the recording.
The solution would be to preserve the magnification of the last view so it remains consistent in reading mode, but place the playback cursor at the correct position somewhere after the beginning of the recording, after generating the last spectrogram that would then have some overlap with the previous one.
related to #203