Assume no state change from previously known state in bgTokenizer (#256)

lib/ace/background_tokenizer.js

@@ -144,6 +144,9 @@ var BackgroundTokenizer = function(tokenizer, editor) {
         } else if (firstRow == 0) {
             state = "start";
             doCache = true;
+        } else if (this.lines.length > 0) {
+            // Guess that we haven't changed state.
+            state = this.lines[this.lines.length-1].state;
         }
 
         var lines = this.doc.getLines(firstRow, lastRow);