Expose the mapping from subset code to unicode

This lets us more easily build ToUnicode cmaps in PDF's

lib/ttfunk/subset/base.rb

@@ -18,6 +18,14 @@ def initialize(original)
         @original = original
       end
 
+      def unicode?
+        false
+      end
+
+      def to_unicode_map
+        {}
+      end
+
       def encode(options={})
         cmap_table = new_cmap_table(options)
         glyphs = collect_glyphs(original_glyph_ids)

lib/ttfunk/subset/unicode.rb

@@ -9,6 +9,14 @@ def initialize(original)
         @subset = Set.new
       end
 
+      def unicode?
+        true
+      end
+
+      def to_unicode_map
+        @subset.inject({}) { |map, code| map[code] = code; map }
+      end
+
       def use(character)
         @subset << character
       end

lib/ttfunk/subset/unicode_8bit.rb

@@ -11,6 +11,14 @@ def initialize(original)
         @next = 0x21 # apparently, PDF's don't like to use chars between 0-31
       end
 
+      def unicode?
+        true
+      end
+
+      def to_unicode_map
+        @subset.dup
+      end
+
       def use(character)
         if !@unicodes.key?(character)
           @subset[@next] = character