support calling rand without arguments.

which is shorter than Math.random() and more consistent if rand is used
in the same places.

js/pixl/public/trixl.html

@@ -109,10 +109,14 @@
		}
		var rand = function(lo, hi) {
			var both = hi !== undefined,
			    l = both ? lo : 0,
			    h = both ? hi : lo;
			return l + Math.round(Math.random() * (h - l));
			switch (arguments.length) {
			case 0:
				return Math.random();
			case 1:
				return Math.round(Math.random() * lo);
			default:
				return lo + Math.round(Math.random() * (hi - lo));
			}
		}
		var rgbFromCss = function(color) {