Improve the handling of `Encoding` dictionary, with `Differences` array, in `PartialEvaluator_preEvaluateFont`

I recently happened to look at the code I wrote for PR 5964, which fixed [bug 1157493](https://bugzilla.mozilla.org/show_bug.cgi?id=1157493), and I quickly realized that the solution is way too simplistic.
The fact that only using the `length` of a `Differences` array worked seems more like a happy accident for a particular set of font data, but could just as easily be incorrect for other PDF files.

Note that in practice, the case where the `Encoding` entry is a regular `Dict` (and not a `Ref` or `Name`) is very rare, hence I don't think that we really need to worry about having to reparse this data.
Also, the performance of this code-block is quite a bit better by updating the `hash` with the data from the *entire* `Differences` array, instead of at every loop iteration.

src/core/evaluator.js

@@ -2189,11 +2189,19 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
               hash.update(entry.name);
             } else if (isRef(entry)) {
               hash.update(entry.toString());
-            } else if (isArray(entry)) { // 'Differences' entry.
-              // Ideally we should check the contents of the array, but to avoid
-              // parsing it here and then again in |extractDataStructures|,
-              // we only use the array length for now (fixes bug1157493.pdf).
-              hash.update(entry.length.toString());
+            } else if (isArray(entry)) {
+              // 'Differences' array (fixes bug1157493.pdf).
+              var diffLength = entry.length, diffBuf = new Array(diffLength);
+
+              for (var j = 0; j < diffLength; j++) {
+                var diffEntry = entry[j];
+                if (isName(diffEntry)) {
+                  diffBuf[j] = diffEntry.name;
+                } else if (isNum(diffEntry) || isRef(diffEntry)) {
+                  diffBuf[j] = diffEntry.toString();
+                }
+              }
+              hash.update(diffBuf.join());
             }
           }
         }