Skip mapping of CIDFontType2 glyphs when the font either has a |IdentityToUnicodeMap| or a |toUnicodeMap| with 65536 elements (issue 5677)

This patch slightly extends the heuristics used when trying to skip mapping of missing glyphs.

Fixes 5677.

test/pdfs/.gitignore

@@ -25,6 +25,7 @@
 !issue4630.pdf
 !issue5202.pdf
 !issue5280.pdf
+!issue5677.pdf
 !issue5954.pdf
 !alphatrans.pdf
 !devicen.pdf

test/test_manifest.json

@@ -558,6 +558,13 @@
        "link": false,
        "type": "load"
     },
+    {  "id": "issue5677",
+       "file": "pdfs/issue5677.pdf",
+       "md5": "c9101578fcb806269145132724d24ac1",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue5954",
        "file": "pdfs/issue5954.pdf",
        "md5": "4f60ec0d9bbeec845b681242b8982361",