For embedded Type1 fonts without included ToUnicode/Encoding data, attempt to improve text selection by using the builtInEncoding to amend the toUnicode map (issue 6901, issue 7182, issue 7217, bug 917796, bug 1242142)

Note that in order to prevent any possible issues, this patch does *not* try to amend the `toUnicode` data for Type1 fonts that contain either `ToUnicode` or `Encoding` entries in the font dictionary.

Fixes, or at least improves, issues/bugs such as e.g. 6658, 6901, 7182, 7217, bug 917796, bug 1242142.

test/pdfs/.gitignore

@@ -22,6 +22,7 @@
 !issue5808.pdf
 !issue6204.pdf
 !issue6782.pdf
+!issue6901.pdf
 !issue6961.pdf
 !issue6962.pdf
 !issue7020.pdf

test/test_manifest.json

@@ -1220,6 +1220,20 @@
        "link": false,
        "type": "text"
     },
+    {  "id": "issue6901-eq",
+       "file": "pdfs/issue6901.pdf",
+       "md5": "1a0604b1a7a3aaf2162b425a9a84230b",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
+    {  "id": "issue6901-text",
+       "file": "pdfs/issue6901.pdf",
+       "md5": "1a0604b1a7a3aaf2162b425a9a84230b",
+       "rounds": 1,
+       "link": false,
+       "type": "text"
+    },
     {  "id": "issue6962",
        "file": "pdfs/issue6962.pdf",
        "md5": "d40e871ecca68baf93114bd28c782148",