Improve text-selection for Type3 fonts with bogus /FontBBox-entries (issue 14999)

This extends PR 13461, by also building a fallback bounding box for Type3 fonts that contain a much too small /FontBBox-entry.

*Please note:* While this patch improves things overall, copy-and-pasting still doesn't work perfectly for this document. In particular the lowercase letter "c" cannot be selected/copied, however this can be reproduced in both Adobe Reader and PDFium (in Google Chrome) too, which is caused by a lack of proper /ToUnicode-data in the PDF document.

test/pdfs/.gitignore

@@ -121,6 +121,7 @@
 !issue13916.pdf
 !issue14023.pdf
 !issue14438.pdf
+!issue14999_reduced.pdf
 !bad-PageLabels.pdf
 !decodeACSuccessive.pdf
 !issue13003.pdf

test/test_manifest.json

@@ -2848,6 +2848,12 @@
        "link": false,
        "type": "text"
     },
+    {  "id": "issue14999",
+       "file": "pdfs/issue14999_reduced.pdf",
+       "md5": "a4e664e734f6869aa66245e72d448874",
+       "rounds": 1,
+       "type": "text"
+    },
     {  "id": "issue6901-eq",
        "file": "pdfs/issue6901.pdf",
        "md5": "1a0604b1a7a3aaf2162b425a9a84230b",