Fills-out implied positions in color-stops. This function is useful for
old Webkit gradient definitions
function fillImpliedPositions(colorStops) {
		var from = 0;
		
		_.each(colorStops, function(cs, i) {
			// make sure that first and last positions are defined
			if (!i)
				return cs.position = cs.position || 0;
			
			if (i == colorStops.length - 1 && !('position' in cs))
				cs.position = 1;
			
			if ('position' in cs) {
				var start = colorStops[from].position || 0;
				var step = (cs.position - start) / (i - from);
				_.each(colorStops.slice(from, i), function(cs2, j) {
					cs2.position = start + step * j;
				});
				
				from = i;
			}
		});
	}