To mitigate concurrent rendering, the waveform cache adds a random
range of pixels centered around the visible waveform.
Alignment is using integer half_width = width_samples / 2;
This always aligns the left-edge to the left-most cairo-pixel.
This fixes an issue with moving moiree patterns in waveforms when
zooming vertically (which invalidates the cache and uses a
different random number of pixels),
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