Add global caching, for /Resources without blend modes, and use it to reduce repeated fetching/parsing in PartialEvaluator.hasBlendModes

The `PartialEvaluator.hasBlendModes` method is necessary to determine if there's any blend modes on a page, which unfortunately requires *synchronous* parsing of the /Resources of each page before its rendering can start (see the "StartRenderPage"-message).
In practice it's not uncommon for certain /Resources-entries to be found on more than one page (referenced via the XRef-table), which thus leads to unnecessary re-fetching/re-parsing of data in `PartialEvaluator.hasBlendModes`.

To improve performance, especially in pathological cases, we can cache /Resources-entries when it's absolutely clear that they do not contain *any* blend modes at all[1]. This way, subsequent `PartialEvaluator.hasBlendModes` calls can be made significantly more efficient.

This patch was tested using the PDF file from issue 6961, i.e. https://github.com/mozilla/pdf.js/files/121712/test.pdf:
```
[
    {  "id": "issue6961",
       "file": "../web/pdfs/issue6961.pdf",
       "md5": "a80e4357a8fda758d96c2c76f2980b03",
       "rounds": 100,
       "type": "eq"
    }
]
```

which gave the following results when comparing this patch against the `master` branch:
```
-- Grouped By browser, page, stat --
browser | page | stat         | Count | Baseline(ms) | Current(ms) |  +/- |     %  | Result(P<.05)
------- | ---- | ------------ | ----- | ------------ | ----------- | ---- | ------ | -------------
firefox | 0    | Overall      |   100 |         1034 |         555 | -480 | -46.39 |        faster
firefox | 0    | Page Request |   100 |          489 |           7 | -482 | -98.67 |        faster
firefox | 0    | Rendering    |   100 |          545 |         548 |    2 |   0.45 |
firefox | 1    | Overall      |   100 |          912 |         428 | -484 | -53.06 |        faster
firefox | 1    | Page Request |   100 |          487 |           1 | -486 | -99.77 |        faster
firefox | 1    | Rendering    |   100 |          425 |         427 |    2 |   0.51 |
```

---
[1] In the case where blend modes *are* found, it becomes a lot more difficult to know if it's generally safe to skip them. Hence we don't cache anything in that case, however note that most document/pages do not utilize blend modes anyway.

Author: Snuffleupagus

src/core/document.js

@@ -76,6 +76,7 @@ class Page {
     fontCache,
     builtInCMapCache,
     globalImageCache,
+    nonBlendModesSet,
   }) {
     this.pdfManager = pdfManager;
     this.pageIndex = pageIndex;
@@ -85,6 +86,7 @@ class Page {
     this.fontCache = fontCache;
     this.builtInCMapCache = builtInCMapCache;
     this.globalImageCache = globalImageCache;
+    this.nonBlendModesSet = nonBlendModesSet;
     this.evaluatorOptions = pdfManager.evaluatorOptions;
     this.resourcesPromise = null;
 
@@ -312,7 +314,10 @@ class Page {
       const opList = new OperatorList(intent, sink);
 
       handler.send("StartRenderPage", {
-        transparency: partialEvaluator.hasBlendModes(this.resources),
+        transparency: partialEvaluator.hasBlendModes(
+          this.resources,
+          this.nonBlendModesSet
+        ),
         pageIndex: this.pageIndex,
         intent,
       });
@@ -917,6 +922,7 @@ class PDFDocument {
         fontCache: catalog.fontCache,
         builtInCMapCache: catalog.builtInCMapCache,
         globalImageCache: catalog.globalImageCache,
+        nonBlendModesSet: catalog.nonBlendModesSet,
       });
     }));
   }

src/core/evaluator.js

@@ -239,12 +239,15 @@ class PartialEvaluator {
     return newEvaluator;
   }
 
-  hasBlendModes(resources) {
+  hasBlendModes(resources, nonBlendModesSet) {
     if (!(resources instanceof Dict)) {
       return false;
     }
+    if (resources.objId && nonBlendModesSet.has(resources.objId)) {
+      return false;
+    }
 
-    const processed = new RefSet();
+    const processed = new RefSet(nonBlendModesSet);
     if (resources.objId) {
       processed.put(resources.objId);
     }
@@ -344,6 +347,15 @@ class PartialEvaluator {
         }
       }
     }
+
+    if (processed.size > 0) {
+      // When no blend modes exist, there's no need check any of the parsed
+      // `Ref`s again for subsequent pages. This helps reduce redundant
+      // `XRef.fetch` calls for some documents (e.g. issue6961.pdf).
+      processed.forEach(ref => {
+        nonBlendModesSet.put(ref);
+      });
+    }
     return false;
   }
 

src/core/obj.js

@@ -76,6 +76,7 @@ class Catalog {
     this.builtInCMapCache = new Map();
     this.globalImageCache = new GlobalImageCache();
     this.pageKidsCountCache = new RefSetCache();
+    this.nonBlendModesSet = new RefSet();
   }
 
   get version() {
@@ -937,6 +938,7 @@ class Catalog {
     clearPrimitiveCaches();
     this.globalImageCache.clear(/* onlyData = */ manuallyTriggered);
     this.pageKidsCountCache.clear();
+    this.nonBlendModesSet.clear();
 
     const promises = [];
     this.fontCache.forEach(function (promise) {

src/core/primitives.js

@@ -277,8 +277,20 @@ var Ref = (function RefClosure() {
 // The reference is identified by number and generation.
 // This structure stores only one instance of the reference.
 class RefSet {
-  constructor() {
-    this._set = new Set();
+  constructor(parent = null) {
+    if (
+      (typeof PDFJSDev === "undefined" ||
+        PDFJSDev.test("!PRODUCTION || TESTING")) &&
+      parent &&
+      !(parent instanceof RefSet)
+    ) {
+      unreachable('RefSet: Invalid "parent" value.');
+    }
+    this._set = new Set(parent && parent._set);
+  }
+
+  get size() {
+    return this._set.size;
   }
 
   has(ref) {
@@ -292,6 +304,16 @@ class RefSet {
   remove(ref) {
     this._set.delete(ref.toString());
   }
+
+  forEach(callback) {
+    for (const ref of this._set) {
+      callback(ref);
+    }
+  }
+
+  clear() {
+    this._set.clear();
+  }
 }
 
 class RefSetCache {