Add a cache to avoid to load several times a local font

On my computer, it takes few tenths of a second to load a local font.
Since a font can be used several times in a document, the cache will
improve performances.

test/unit/annotation_spec.js

@@ -130,6 +130,7 @@ describe("annotation", function () {
       fontCache: new RefSetCache(),
       builtInCMapCache,
       standardFontDataCache: new Map(),
+      systemFontCache: new Map(),
     });
   });