Fix getScaleProvidingLayerForUnit to make it only return a layer that actually has display extents. Modify getVisibleExtentsForUnit to make it more like the behaviour in 3.x: where no layer with display extents is found, use the union of the value extents of layers with the right unit. Partial fix for #1954 Peculiar alignment for Amplitude Follower y-scale in Auto-Align mode.