message: OpenType: Add Anchor table format 3
diff: @@ -1140,6 +1140,8 @@ let optional_value_record = fun (table_start : Pos) => fun (flags : U16) => matc
  ///
  /// - [Microsoft's OpenType Spec: Anchor Tables](https://docs.microsoft.com/en-us/typography/opentype/spec/gpos#anchor-tables)
  let anchor_table = {
  + /// The start of the table
  + table_start <- stream_pos,
  /// Format identifier
  anchor_format <- u16be,
  table <- match anchor_format {
  @@ -1160,7 +1162,18 @@ let anchor_table = {
  anchor_point <- u16be,
  },
  // Anchor Table Format 3: Design Units Plus Device or VariationIndex Tables
  - // TODO
  + 3 => {
  + /// Horizontal value, in design units
  + x_coordinate <- s16be,
  + /// Vertical value, in design units
  + y_coordinate <- s16be,
  + /// Offset to Device table (non-variable font) / VariationIndex table (variable font)
  + /// for X coordinate, from beginning of Anchor table (may be NULL)
  + x_device_offset <- offset16 table_start device_or_variation_index_table,
  + /// Offset to Device table (non-variable font) / VariationIndex table (variable font)
  + /// for Y coordinate, from beginning of Anchor table (may be NULL)
  + y_device_offset <- offset16 table_start device_or_variation_index_table,
  + },
  _ => unknown_table,
  }
  };
  