Amend #1958 - PCA - adjust background color of formula display

The background need to be set twice. Otherwise, it works in the wizard,
but not in the editor.

chemclipse/plugins/org.eclipse.chemclipse.xxd.process.supplier.pca.ui/src/org/eclipse/chemclipse/xxd/process/supplier/pca/ui/swt/PreprocessingSettingsUI.java

@@ -352,6 +352,7 @@ public void paintControl(PaintEvent e) {
 					int x = (int)(dest.width / 2.0f - src.width / 2.0f);
 					int y = (int)(dest.height / 2.0f - src.height / 2.0f);
 					e.gc.drawImage(image, x, y);
+					canvas.setBackground(Display.getDefault().getSystemColor(SWT.COLOR_LIST_BACKGROUND));
 				}
 			}
 		});