fix calculation of line width after join with last child

Signed-off-by: Florian Thienel <florian@thienel.org>
diff --git a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/Line.java b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/Line.java
index 51d0129..9febca6 100644
--- a/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/Line.java
+++ b/org.eclipse.vex.core/src/org/eclipse/vex/core/internal/boxes/Line.java
@@ -95,9 +95,10 @@
 			return false;
 		}
 		final IInlineBox lastChild = children.getLast();
+		final int lastChildOldWidth = lastChild.getWidth();
 		final boolean joined = lastChild.join(box);
 		if (joined) {
-			width += box.getWidth();
+			width += lastChild.getWidth() - lastChildOldWidth;
 		}
 		return joined;
 	}