refactored engine to be a class and turned on best compression for chunky_png

lib/compass/sass_extensions/sprites/engines/chunky_png_engine.rb

@@ -7,20 +7,19 @@ end
 module Compass
   module SassExtensions
     module Sprites
-      module ChunkyPngEngine
-        
-        # Returns a PNG object
+      class ChunkyPngEngine < Compass::SassExtensions::Sprites::Engine
+
         def construct_sprite
-          output_png = ChunkyPNG::Image.new(width, height, ChunkyPNG::Color::TRANSPARENT)
+          @canvas = ChunkyPNG::Image.new(width, height, ChunkyPNG::Color::TRANSPARENT)
           images.each do |image|
             input_png  = ChunkyPNG::Image.from_file(image.file)
             if image.repeat == "no-repeat"
-              output_png.replace! input_png, image.left, image.top
+              canvas.replace! input_png, image.left, image.top
             else
               x = image.left - (image.left / image.width).ceil * image.width
               while x < width do
                 begin
-                  output_png.replace! input_png, x, image.top
+                  canvas.replace! input_png, x, image.top
                   x += image.width 
                 rescue ChunkyPNG::OutOfBounds
                   break;
@@ -28,8 +27,15 @@ module Compass
               end
             end
           end
-          output_png
+        end    
+        
+        def save(filename)
+          if canvas.nil?
+            construct_sprite
+          end
+          canvas.save(filename, :best_compression)
         end
+        
       end
     end
   end