Don't directly access the private `map` in `setGState`, and ensure that we avoid indirect objects

*This patch is based on something I noticed while debugging some of the PDF files in issue 6931.*

In a number of the cases in `setGState`, we're implicitly assuming that we're not dealing with indirect objects (i.e. `Ref`s). See e.g. the 'Font' case, or the various cases where we simply do `gStateObj.push([key, value]);` (since the code in `canvas.js` won't be able to deal with a `Ref` for those cases).

The reason that I didn't use `Dict_forEach` instead, is that it would re-introduce the unncessary closures that PR 5205 removed.

src/core/evaluator.js

@@ -468,11 +468,12 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
                                                    xref, stateManager) {
       // This array holds the converted/processed state data.
       var gStateObj = [];
-      var gStateMap = gState.map;
+      var gStateKeys = gState.getKeys();
       var self = this;
       var promise = Promise.resolve();
-      for (var key in gStateMap) {
-        var value = gStateMap[key];
+      for (var i = 0, ii = gStateKeys.length; i < ii; i++) {
+        var key = gStateKeys[i];
+        var value = gState.get(key);
         switch (key) {
           case 'Type':
             break;
@@ -505,12 +506,11 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
               gStateObj.push([key, false]);
               break;
             }
-            var dict = xref.fetchIfRef(value);
-            if (isDict(dict)) {
-              promise = promise.then(function () {
+            if (isDict(value)) {
+              promise = promise.then(function (dict) {
                 return self.handleSMask(dict, resources, operatorList,
                                         task, stateManager);
-              });
+              }.bind(this, value));
               gStateObj.push([key, true]);
             } else {
               warn('Unsupported SMask type');