Convert uniXXXX glyph names to proper ones when building the charCodeToGlyphId map for TrueType fonts (bug 1132849, issue 6893, issue 6894)

This patch adds a `getUnicodeForGlyph` helper function, which is used to recover Unicode values for non-standard glyph names.

Some PDF generators, e.g. Scribus PDF, use improper `uniXXXX` glyph names which breaks the glyph mapping. We can avoid this by converting them to "standard" glyph names instead.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1132849.
Fixes 6893.
Fixes 6894.

test/pdfs/.gitignore

@@ -32,6 +32,8 @@
 !bug1200096.pdf
 !issue5564_reduced.pdf
 !canvas.pdf
+!bug1132849.pdf
+!issue6894.pdf
 !issue5804.pdf
 !ShowText-ShadingPattern.pdf
 !complex_ttf_font.pdf

test/test_manifest.json

@@ -728,6 +728,20 @@
        "rounds": 1,
        "type": "eq"
     },
+    {  "id": "bug1132849",
+       "file": "pdfs/bug1132849.pdf",
+       "md5": "aedfbead1f8feb35cf2e38b279133b47",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
+    {  "id": "issue6894",
+       "file": "pdfs/issue6894.pdf",
+       "md5": "bb84f2025c11f23cf436170049f81215",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "personwithdog",
        "file": "pdfs/personwithdog.pdf",
        "md5": "cd68fb2ce00dab97801b3e51495b99e3",