Fallback in readCmapTable, instead of using error, for TrueType fonts with unsupported cmap formats (bug 1200096)

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1200096.

The problematic font has a `format 2` cmap, which we've never supported properly. Prior to PR 2606, we were able to fallback to a working state, despite not having proper support for that cmap format.

Obviously the best/correct solution would be to implement actual support for more cmap formats[1]. However, I'm hoping that a simple patch will be OK for now, given that:
 - `format 2` cmaps seem to be quite rare in practice, since this has been broken for 2.5 years before anyone noticed.
 - Having a simple patch will make potential uplifts a lot easier.

[1] See the specification at https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6cmap.html

src/core/fonts.js

@@ -3390,7 +3390,13 @@ var Font = (function FontClosure() {
             });
           }
         } else {
-          error('cmap table has unsupported format: ' + format);
+          warn('cmap table has unsupported format: ' + format);
+          return {
+            platformId: -1,
+            encodingId: -1,
+            mappings: [],
+            hasShortCmap: false
+          };
         }
 
         // removing duplicate entries

test/pdfs/.gitignore

@@ -15,6 +15,7 @@
 !arial_unicode_en_cidfont.pdf
 !asciihexdecode.pdf
 !bug1050040.pdf
+!bug1200096.pdf
 !canvas.pdf
 !complex_ttf_font.pdf
 !extgstate.pdf

test/test_manifest.json

@@ -629,6 +629,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "bug1200096",
+       "file": "pdfs/bug1200096.pdf",
+       "md5": "b6bd8df094b5d511c13ed095d2a07515",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "hudsonsurvey",
        "file": "pdfs/hudsonsurvey.pdf",
        "md5": "bf0e6576a7b6c2fe7485bce1b78e006f",