8356814: LineBreakMeasurer.nextLayout() slower than necessary when no break needed

Reviewed-by: prr, dnguyen, honkar

Author: Daniel Gredler

src/java.desktop/share/classes/sun/font/ExtendedTextSourceLabel.java

@@ -875,7 +875,14 @@ protected int v2l(int index) {
   }
 
   public TextLineComponent getSubset(int start, int limit, int dir) {
-    return new ExtendedTextSourceLabel(source.getSubSource(start, limit-start, dir), decorator);
+    if (start == 0 &&
+        limit == source.getLength() &&
+        (dir == source.getBidiLevel() || dir == TextLineComponent.UNCHANGED)) {
+      return this; // avoid unnecessary object creation and text shaping
+    } else {
+      TextSource subSource = source.getSubSource(start, limit-start, dir);
+      return new ExtendedTextSourceLabel(subSource, decorator);
+    }
   }
 
   public String toString() {