Tweak to zoom slider shading -- don't shade unless viz zoom level is …

…> 1. The calculation will never result in a zoom level of < 1, 1 => whole chromosome. Track hub tracks often have visibility windows > the chromosome length, this was being shaded at zoom level 1.

src/main/java/org/broad/igv/ui/panel/ZoomSliderPanel.java

@@ -214,6 +214,7 @@ private void paintVisibilityThresholds(final Graphics2D transGraphics) {
                     .sorted()
                     .distinct()
                     .map(threshold -> this.getReferenceFrame().calculateZoom(0, threshold))
+                    .filter(z -> z > 1)
                     .toList();
 
             transGraphics.setColor(TRANSPARENT_BLUE);