Audio waveforms and Mixer levels don't reflect true values.

• It seems the audio waveforms are not drawn correctly. See comparison:

Kdenlive:

Audacity:

• The mixer levels in master don't reflect the changes applied. For example if you add a gain or normalization effect in master the level stays the same:

• Would be good if the audio waveform also changes when we apply effects to a clip.

Related to #792 (closed)

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See #1888