Map missing glyphs to the notdef glyph for TrueType (3, 1) fonts (issue 6068)

Fixes 6068.

The most notable issue with the font in question is that the `differences` array contains lots of strange entries (of the type `uniXXXX`, instead of proper glyph names).

src/core/fonts.js

@@ -4225,11 +4225,12 @@ var Font = (function FontClosure() {
               }
             }
             if (!found && properties.glyphNames) {
-              // Try to map using the post table. There are currently no known
-              // pdfs that this fixes.
+              // Try to map using the post table.
               var glyphId = properties.glyphNames.indexOf(glyphName);
               if (glyphId > 0 && hasGlyph(glyphId, -1, -1)) {
                 charCodeToGlyphId[charCode] = glyphId;
+              } else {
+                charCodeToGlyphId[charCode] = 0; // notdef
               }
             }
           }

test/pdfs/.gitignore

@@ -135,4 +135,5 @@
 !issue5909.pdf
 !issue6010_1.pdf
 !issue6010_2.pdf
+!issue6068.pdf
 !issue6081.pdf

test/test_manifest.json

@@ -1137,6 +1137,13 @@
       "rounds": 1,
       "type": "eq"
     },
+    {  "id": "issue6068",
+       "file": "pdfs/issue6068.pdf",
+       "md5": "bbcedb94776b40352729c16940a5b2bd",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "tamreview",
       "file": "pdfs/TAMReview.pdf",
       "md5": "8039aba56790d3597d2bc8c794a51301",