Add local caching of "simple" Graphics State (ExtGState) data in PartialEvaluator.getOperatorList (issue 2813)

This patch will help pathological cases the most, with issue 2813 being a particularily problematic example. While there's only *four* `/ExtGState` resources, there's a total `29062` of `setGState` operators. Even though parsing of a single `/ExtGState` resource is quite fast, having to re-parse them thousands of times does add up quite significantly.

For simplicity we'll only cache "simple" `/ExtGState` resource, since e.g. the general `SMask` case cannot be easily cached (without re-factoring other code, which may have undesirable effects on general parsing).

By caching "simple" `/ExtGState` resource, we thus improve performance by:
 - Not having to fetch/validate/parse the same `/ExtGState` data over and over.
 - Handling of repeated `setGState` operators becomes *synchronous* during the `OperatorList` building, instead of having to defer to the event-loop/microtask-queue since the `/ExtGState` parsing is done asynchronously.

---

Obviously I had intended to include (standard) benchmark results with this patch, but for reasons I don't understand the test run-time (even with `master`) of the document in issue 2813 is *a lot* slower than in the development viewer (making normal benchmarking infeasible).
However, testing this manually in the development viewer (using `pdfBug=Stats`) shows a *reduction* of `~10 %` in the rendering time of the PDF document in issue 2813.

src/core/image_utils.js

@@ -113,6 +113,27 @@ class LocalFunctionCache extends BaseLocalCache {
   }
 }
 
+class LocalGStateCache extends BaseLocalCache {
+  set(name, ref = null, data) {
+    if (!name) {
+      throw new Error('LocalGStateCache.set - expected "name" argument.');
+    }
+    if (ref) {
+      if (this._imageCache.has(ref)) {
+        return;
+      }
+      this._nameRefMap.set(name, ref);
+      this._imageCache.put(ref, data);
+      return;
+    }
+    // name
+    if (this._imageMap.has(name)) {
+      return;
+    }
+    this._imageMap.set(name, data);
+  }
+}
+
 class GlobalImageCache {
   static get NUM_PAGES_THRESHOLD() {
     return shadow(this, "NUM_PAGES_THRESHOLD", 2);
@@ -210,5 +231,6 @@ export {
   LocalImageCache,
   LocalColorSpaceCache,
   LocalFunctionCache,
+  LocalGStateCache,
   GlobalImageCache,
 };