Deep duplicate calls when making a copy of the call stack

Some prawn methods mutate the arguments passed in to them, so to combot that
we need to deep duplicate the arguments whenever we duplicate the call stacks.

This fixes the reported problem in #49.

lib/prawn/svg/elements.rb

@@ -2,6 +2,8 @@ module Prawn::SVG::Elements
   COMMA_WSP_REGEXP = /(?:\s+,?\s*|,\s*)/
 end
 
+require 'prawn/svg/elements/call_duplicator'
+
 %w(base depth_first_base root container viewport style text text_component line polyline polygon circle ellipse rect path use image gradient marker ignored).each do |filename|
   require "prawn/svg/elements/#{filename}"
 end

lib/prawn/svg/elements/base.rb

@@ -1,6 +1,8 @@
 class Prawn::SVG::Elements::Base
   extend Forwardable
 
+  include Prawn::SVG::Elements::CallDuplicator
+
   include Prawn::SVG::Calculators::Pixels
 
   include Prawn::SVG::Attributes::Transform
@@ -106,7 +108,7 @@ def append_calls_to_parent
   end
 
   def add_calls_from_element(other)
-    @calls.concat other.base_calls
+    @calls.concat duplicate_calls(other.base_calls)
   end
 
   def new_call_context_from_base

lib/prawn/svg/elements/call_duplicator.rb

@@ -0,0 +1,37 @@
+#
+# Unfortunately, prawn mutates arguments passed in to it.
+# When we make a copy of one of the call stacks, we need to make a deep
+# duplicate of it so that the first time prawn mutates the arguments, it
+# won't affect the subsequent calls.
+#
+module Prawn::SVG::Elements::CallDuplicator
+  private
+
+  def duplicate_calls(calls)
+    calls.map { |call| duplicate_call(call) }
+  end
+
+  def duplicate_call(call)
+    [call[0], duplicate_array(call[1]), duplicate_calls(call[2])]
+  end
+
+  def duplicate_array(array)
+    array.map do |value|
+      case value
+      when Array then duplicate_array(value)
+      when Hash  then duplicate_hash(value)
+      else            value
+      end
+    end
+  end
+
+  def duplicate_hash(hash)
+    hash.each.with_object({}) do |(key, value), result|
+      result[key] = case value
+                    when Array then duplicate_array(value)
+                    when Hash  then duplicate_hash(value)
+                    else            value
+                    end
+    end
+  end
+end