fix centered text alignment: take left invisible gap into account

Signed-off-by: Florian Thienel <florian@thienel.org>
diff --git a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/LineArrangement.java b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/LineArrangement.java
index 772a68c..eba1fbc 100644
--- a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/LineArrangement.java
+++ b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/LineArrangement.java
@@ -193,7 +193,7 @@
 	private int alignmentOffset(final Graphics graphics, final Line line, final TextAlign textAlign) {
 		switch (textAlign) {
 		case CENTER:
-			return (width - line.getWidth()) / 2;
+			return (width - line.getWidth() + line.getInvisibleGapLeft(graphics) + line.getInvisibleGapRight(graphics)) / 2 - line.getInvisibleGapLeft(graphics);
 		case RIGHT:
 			return width - line.getWidth() + line.getInvisibleGapRight(graphics);
 		default: