Add support for TrueType format 12 cmaps (issue 14881)

This is, as far as I can tell, the first case we've seen of a format 12 `cmap`.
Please see https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6cmap.html

test/pdfs/.gitignore

@@ -18,6 +18,7 @@
 !issue2391-2.pdf
 !issue14046.pdf
 !issue7891_bc1.pdf
+!issue14881.pdf
 !issue3214.pdf
 !issue4665.pdf
 !issue4684.pdf

test/test_manifest.json

@@ -4630,6 +4630,12 @@
        "password": "\u00E6\u00F8\u00E5",
        "about": "The password (æøå) is UTF8 encoded."
     },
+    {  "id": "issue14881",
+       "file": "pdfs/issue14881.pdf",
+       "md5": "1aa3f2c7929eaf4502fcf6488ff31b5f",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "High-Pressure-Measurement-WP-001287",
        "file": "pdfs/High-Pressure-Measurement-WP-001287.pdf",
        "md5": "aeba7e47bbe50cbf08bb8bdff78fec8c",