Fix artifact lines in Forms UI Section headers #3381

Section headers show artifacts lines because a previous change
accidentally changed the operation for filling the background to use a
height of 1 instead of the actual height to be filled.

Fixes #3381

Author: HeikoKlare

bundles/org.eclipse.ui.forms/src/org/eclipse/ui/internal/forms/widgets/FormImages.java

@@ -315,7 +315,7 @@ public Image createImage(boolean returnMissingImageOnError, Device device) {
 			Color color2 = new Color(device, fRGBs[1]);
 			final ImageGcDrawer imageGcDrawer = (gc, width, height) -> {
 				gc.setBackground(color1);
-				gc.fillRectangle(0, 0, width, width);
+				gc.fillRectangle(0, 0, width, height);
 				gc.setForeground(color2);
 				gc.setBackground(color1);
 				gc.fillGradientRectangle(0, fMarginHeight + 2, 1, fTheight - 2, true);