[TrueType] Recover from a missing "glyf" table by replacing it with dummy data, utilizing the existing code in `sanitizeGlyphLocations`

It seems to be fairly common for OCR software to include incomplete TrueType fonts, notable missing the "glyf" table, in PDF files. Since we currently reject such fonts, the result is that text-selection/copying is broken.

This patch contains a suggested approach to try and use these kind of broken fonts, by using existing code in `sanitizeGlyphLocations` to replace a missing "glyf" table with dummy data.

Fixes 4684.
Fixes 6007.
Fixes 6829.

src/core/fonts.js

@@ -4154,8 +4154,16 @@ var Font = (function FontClosure() {
         delete tables['cvt '];
         this.isOpenType = true;
       } else {
-        if (!tables.glyf || !tables.loca) {
+        if (!tables.loca) {
-          error('Required "glyf" or "loca" tables are not found');
+          error('Required "loca" table is not found');
+        }
+        if (!tables.glyf) {
+          warn('Required "glyf" table is not found -- trying to recover.');
+          // Note: We use `sanitizeGlyphLocations` to add dummy glyf data below.
+          tables.glyf = {
+            tag: 'glyf',
+            data: new Uint8Array(0),
+          };
         }
         this.isOpenType = false;
       }

test/pdfs/.gitignore

@@ -10,6 +10,7 @@
 !issue2391-1.pdf
 !issue2391-2.pdf
 !issue4665.pdf
+!issue4684.pdf
 !issue5801.pdf
 !issue5946.pdf
 !issue5972.pdf

test/test_manifest.json

@@ -1789,6 +1789,14 @@
       "type": "eq",
       "about": "Has a multi-byte char codes."
     },
+    {  "id": "issue4684-text",
+       "file": "pdfs/issue4684.pdf",
+       "md5": "af5056fcdfb08bd7adc1710d36e4b5b5",
+       "link": false,
+       "rounds": 1,
+       "type": "text",
+       "about": "Invisible (and broken) TrueType font used for text-selection."
+    },
     {  "id": "issue1912",
       "file": "pdfs/issue1912.pdf",
       "md5": "15305b7c2cba971e7423de3f6ad38fef",