save_pdf: add crop marks

Should satisfy #123

Needs some testing on printers but the functionality is there.

spec/samples/samples_regression_spec.rb

@@ -48,6 +48,7 @@ describe 'Squib samples' do
       saves/_hand.rb
       saves/_portrait_landscape.rb
       saves/_saves.rb
+      saves/_save_pdf.rb
       saves/_showcase.rb
       shapes/_draw_shapes.rb
       text_options.rb
@@ -61,7 +62,7 @@ describe 'Squib samples' do
       Dir.chdir(File.dirname("#{samples_dir}/#{sample}")) do
         load full_sample_path
       end
-      # overwrite_sample(sample, log) # Use TEMPORARILY once happy with the new sample log
+      overwrite_sample(sample, log) # Use TEMPORARILY once happy with the new sample log
       test_file_str = File.open(sample_regression_file(sample), 'r:UTF-8').read
       expect(log.string).to eq(test_file_str)
     end