Support CMap-data with only strings, when parsing TrueType composite fonts (bug 920426)

In the referenced bug, the embedded fonts contain custom CMap-data that only include strings. Note how for embedded composite TrueType fonts we're using the CMap-data when building the glyph mapping, and currently we end up with a completely empty map because the code expects only CID *numbers*.
Furthermore, just fixing the glyph mapping alone isn't sufficient to fully address the bug, since we also need to consider this "special" kind of CMap-data when looking up glyph widths.

test/pdfs/.gitignore

@@ -238,6 +238,7 @@
 !bug900822.pdf
 !bug1392647.pdf
 !issue918.pdf
+!bug920426.pdf
 !issue1905.pdf
 !issue2833.pdf
 !issue2931.pdf

test/test_manifest.json

@@ -154,6 +154,12 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "bug920426",
+       "file": "pdfs/bug920426.pdf",
+       "md5": "2fd118b44513afb22509c3584388cc29",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "issue13343",
        "file": "pdfs/issue13343.pdf",
        "md5": "f8bf1888839e15254555092c504e1900",