Eraser actually erases pixels back to transparent now instead of just drwaing white over the canvas
Added a quick and dirty background image to the html canvas

By no means is this image permanent, it's just the first one I saw from a google search. With this change the canvas has an image background that zooms and pans along with the canvas. Erasing does not erase the background.

If we don't want the background to zoom and pan with the drawing then we should put the css background image on the #viewport element instead.