Properly set CFF encoding for subset font

CFF top dict could skip entries that have default values. We have to
specifically set encoding in subset fonts because it's very unlikely
we'll be using the default encoding.

lib/ttfunk/table/cff.rb

@@ -58,6 +58,9 @@ def tag
       #   TTFunk::Subset::Unicode, TTFunk::Subset::Unicode8Bit]
       # @return [TTFunk::EncodedString]
       def encode(subset)
+        # Make sure TopDict has an entry for encoding so it could be properly replaced
+        top_index[0][TopDict::OPERATORS[:encoding]] = 0
+
         EncodedString.new do |result|
           result.concat(
             header.encode,

lib/ttfunk/table/cff/dict.rb

@@ -44,6 +44,14 @@ def [](operator)
           @dict[operator]
         end
 
+        # Add dict entry.
+        #
+        # @param operator [Integer] Entry operator. Must be in range 0..255. Wide operators must be in range 1200..1455.
+        # @param operands [Array<Integer, TTFunk::SciForm>]
+        def []=(operator, *operands)
+          @dict[operator] = Array(*operands)
+        end
+
         # Iterate over dict entries.
         #
         # @yieldparam key [Integer]
@@ -59,7 +67,8 @@ def each(&block)
         #
         # @return [String]
         def encode
-          map do |(operator, operands)|
+          sort_by(&:first)
+          .map do |(operator, operands)|
             operands.map { |operand| encode_operand(operand) }.join +
               encode_operator(operator)
           end.join

spec/ttfunk/table/cff/dict_spec.rb

@@ -79,4 +79,34 @@
       described_class::TooManyOperandsError
     )
   end
+
+  it 'allows addition of entries' do
+    dict = described_class.new(TestFile.new(StringIO.new('')), 0, 0)
+
+    dict[1] = 42
+    dict[1201] = [43, 44]
+
+    expect(dict.encode).to eq("\xB5\x01\xb6\xb7\x0c\x01".b)
+  end
+
+  it 'allows replacement of entries' do
+    dict = described_class.new(TestFile.new(StringIO.new("\xB5\x01\xb6\xb7\x0c\x01".b)), 0, 6)
+
+    dict[1] = 0
+
+    expect(dict.encode).to eq("\x8b\x01\xb6\xb7\x0c\x01".b)
+  end
+
+  it 'uses a stable encoding order' do
+    dict1 = described_class.new(TestFile.new(StringIO.new('')), 0, 0)
+    dict2 = described_class.new(TestFile.new(StringIO.new('')), 0, 0)
+
+    dict1[1] = 1
+    dict1[2] = 2
+
+    dict2[2] = 2
+    dict2[1] = 1
+
+    expect(dict1.encode).to eq(dict2.encode)
+  end
 end