turned transparant background option off by default in image export

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@886 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/userinterface/graph/GUIImageExportDialog.java

@@ -95,7 +95,7 @@ public class GUIImageExportDialog extends JDialog implements DocumentListener
 		
 		this.imageTypeInputField.setSelectedIndex(defaultImageType);
 		
-		this.alphaInputField.setSelected(defaultImageType == PNG);
+		this.alphaInputField.setSelected(false);
 		this.alphaInputField.setEnabled(defaultImageType == PNG);
 		this.alphaInputLabel.setEnabled(defaultImageType == PNG);