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...

test/pdfs/.gitignore

@@ -225,6 +225,7 @@
 !issue840.pdf
 !160F-2019.pdf
 !issue4402_reduced.pdf
+!issue16263.pdf
 !issue845r.pdf
 !issue3405r.pdf
 !issue14130.pdf

test/test_manifest.json

@@ -4721,6 +4721,12 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue16263",
+       "file": "pdfs/issue16263.pdf",
+       "md5": "93c3e7884f1f2d072442898127e0a6fd",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "issue2006",
       "file": "pdfs/issue2006.pdf",
       "md5": "71ec73831ece9b508ad20efa6ff28642",