Improve glyph mapping for non-embedded composite standard fonts with a /CIDToGIDMap (issue 11915)

*Please note:* All of this feels very handwavy, but at least it passes all tests locally. Hopefully we have enough tests for this part of the font code.

For non-embedded composite standard fonts with an "incomplete" /CIDToGIDMap, we'll now fallback to an *explicitly defined* /ToUnicode map even when that one happens to be an /Identity-H or /Identity-V map.

The `Font.fallbackToSystemFont` method is unfortunately getting more and more special-cases, however that might be unavoidable given all the weird non-embedded fonts found in the wild :-(

test/pdfs/.gitignore

@@ -355,6 +355,7 @@
 !tensor-allflags-withfunction.pdf
 !issue10084_reduced.pdf
 !issue4246.pdf
+!issue11915.pdf
 !js-authors.pdf
 !issue4461.pdf
 !issue4573.pdf

test/test_manifest.json

@@ -2365,6 +2365,12 @@
        "link": false,
        "type": "text"
     },
+    {  "id": "issue11915",
+       "file": "pdfs/issue11915.pdf",
+       "md5": "c4f890b4a5b6f21653421856335181d5",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "issue10614",
        "file": "pdfs/issue10614.pdf",
        "md5": "c41da60ce9af100cb78e1c2a6ba18232",