add signature check to Font.isFont

It is possible that we end up with multiple versions of the Font object
when pdfjs is bundled via e.g. webpack. To still accept such fonts,
the `isFont` method is extended by a signature check (tests whether
all necessary public methods are available) as a fallback.

Refs rkusa#182

lib/font/base.js

@@ -2,7 +2,17 @@
 
 class Font {
   static isFont(font) {
-    return font && font instanceof Font
+    return font && (font instanceof Font || (
+      typeof font === 'object'
+      && typeof font.encode === 'function'
+      && typeof font.stringWidth === 'function'
+      && typeof font.lineHeight === 'function'
+      && typeof font.ascent === 'function'
+      && typeof font.descent === 'function'
+      && typeof font.underlinePosition === 'function'
+      && typeof font.underlineThickness === 'function'
+      && typeof font.write === 'function'
+    ))
   }
 }