Fallback to using the name table to infer the encoding for TrueType fonts missing such data (issue 15910)

The relevant TrueType font is missing both /ToUnicode *and* /Encoding entires, either of which would have prevented the (current) broken textLayer rendering.
My first idea was that we could use the `post` table in the TrueType font, see https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6post.html, to get the actual glyphNames and amend the fallback ToUnicode-map that way. Unfortunately that didn't work, since the `post` table only contained ".notdef" and "" (i.e. empty string) entries.

Instead we try to use the `name` table in the TrueType font, see https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6name.html, to determine if the platform is Windows and thus fallback to generate a ToUnicode-map from the `WinAnsiEncoding`.

test/pdfs/.gitignore

@@ -242,6 +242,7 @@
 !bug1108301.pdf
 !issue10301.pdf
 !bug1157493.pdf
+!issue15910.pdf
 !issue4260_reduced.pdf
 !bug1250079.pdf
 !bug1473809.pdf

test/test_manifest.json

@@ -3056,6 +3056,12 @@
        "link": true,
        "type": "eq"
     },
+    {  "id": "issue15910",
+       "file": "pdfs/issue15910.pdf",
+       "md5": "6429d8490e11e226b1cbdf2033c04237",
+       "rounds": 1,
+       "type": "text"
+    },
     {  "id": "issue7872",
        "file": "pdfs/issue7872.pdf",
        "md5": "81781dfecfcb7e9cd9cc7e60f8b747b7",