Return OK_STATUS from the semantic highlighting job when the editor…

… is dirty.

Previously, we announced that the job finishes asynchronously (`ASYNC_FINISH`),
but forgot to call `done` if the editor was dirty (and no UI asyncExec was
performed at all). This lead in turn to the job manager thinking that the job
is still running and not scheduling another run, ever, meaning the loss of
semantic highlighting.

Fixed #1001536.

org.scala-ide.sdt.core/src/scala/tools/eclipse/semantichighlighting/Presenter.scala

@@ -131,20 +131,22 @@ class Presenter(
 
         if (damagedRegion.getLength > 0) {
           val sortedPositions = newPositions.sorted.toArray
-          runPositionsUpdateInUiThread(sortedPositions, damagedRegion)
-          Job.ASYNC_FINISH
+          /* if the positions held by the `positionsTracker` have changed, then 
+           * it's useless to proceed because the `newPositions` have computed on a  
+           * not up-to-date compilation unit. Let the next reconciler run take care 
+           * of re-computing the correct positions with the up-to-date content. 
+           */
+          if (!positionsTracker.isDirty) {
+            runPositionsUpdateInUiThread(sortedPositions, damagedRegion)
+            Job.ASYNC_FINISH
+          } else Status.OK_STATUS
         }
         else Status.OK_STATUS
       }(Status.OK_STATUS)
     }
 
     private def runPositionsUpdateInUiThread(newPositions: Array[Position], damagedRegion: IRegion): Unit =
-      /* if the positions held by the `positionsTracker` have changed, then 
-       * it's useless to proceed because the `newPositions` have computed on a  
-       * not up-to-date compilation unit. Let the next reconciler run take care 
-       * of re-computing the correct positions with the up-to-date content. 
-       */
-      if (!positionsTracker.isDirty) uiThread.asyncExec {
+      uiThread.asyncExec {
         try {
           setThread(uiThread.get)
           if (!positionsTracker.isDirty) {