Waveform drawing bug on Linux

[j-bresson]

With the latest changes on waveform drawing (not in v1.5)

From @andersvi

all well first time the editor is opened - zooming, resizing etc. works great.

Next time, the original waveforms are there, but there's one single waveform (one channel?) added in the middle of the editor-view, seem to be every second vertical drawing line or something is not draw

However, if zooming in slightly, this strange one disappears, only to reappear if zooming out to full view again.