"delete" pixls by overdrawing them in white.

js/pixl/index.html

					y: pixl.window.h / 2 + (world_pos.y - pixl.pos.y) * pixl.size};
		}
		pixl.at = function(pos) {
			var p = pixl.world[pos.x + "," + pos.y];
			return p !== undefined && p.color !== "white";
		}
		pixl.draw_pixl = function(pos, color) {
			var color = color || "black";
			pixl.ctx.fillStyle = color;
		pixl.stage.addEventListener("mouseup", function(ev) {
			if (pixl.drag.current === undefined) {
				var ev = polyfill.movement(ev);
				pixl.draw_pixl(pixl.to_world({x: ev.clientX, y: ev.clientY}));
				var world_pos = pixl.to_world({x: ev.clientX, y: ev.clientY});
				pixl.draw_pixl(world_pos, pixl.at(world_pos) ? "white" : "black");
			} else {
				console.error("drag not implemented");
			}