added layer object from which polygon_layer and image_layer can inherit some basic functionality
added image_layer to load and render external images
rendered canvas is not stored in imageobject anymore because canvas renderes layercanvas directly
instead of image in render method it will use drawCanvas and draw the canvases of the layers directly && added prepareToRender function that will wait until all layeres have been rendered before rendering canvas && added globaleComposite change listener that will trigger a rerender of canvas
added new added files to build script
modified canvastest