Attempt to also cache images at the "page"-level (issue 16263)

Currently we have two separate image-caches on the worker-thread:
 - A local one, which is unique to each `PartialEvaluator.getOperatorList` invocation. This one caches both names *and* references, since image-resources may be accessed in either way.
 - A global one, which applies to the entire PDF documents and all its pages. This one only caches references, since nothing else would work.

This patch introduces a third image-cache, which essentially sits "between" the two existing ones. The new `RegionalImageCache`[1] will be usable throughout a `PartialEvaluator` instance, and consequently it *only* caches references, which thus allows us to keep track of repeated image-resources found in e.g. different /Form and /SMask objects.

---
[1] For lack of a better word, since naming things is hard...

src/core/image_utils.js

@@ -156,6 +156,22 @@ class LocalTilingPatternCache extends BaseLocalCache {
   }
 }
 
+class RegionalImageCache extends BaseLocalCache {
+  constructor(options) {
+    super({ onlyRefs: true });
+  }
+
+  set(name = null, ref, data) {
+    if (!ref) {
+      throw new Error('RegionalImageCache.set - expected "ref" argument.');
+    }
+    if (this._imageCache.has(ref)) {
+      return;
+    }
+    this._imageCache.put(ref, data);
+  }
+}
+
 class GlobalImageCache {
   static get NUM_PAGES_THRESHOLD() {
     return shadow(this, "NUM_PAGES_THRESHOLD", 2);
@@ -288,4 +304,5 @@ export {
   LocalGStateCache,
   LocalImageCache,
   LocalTilingPatternCache,
+  RegionalImageCache,
 };