function addLineNumbers(string) {
    var chunks = string.split('\n');
    for (var i = 0, il = chunks.length; i < il; i ++) {
        // Chrome reports shader errors on lines
        // starting counting from 1
        chunks[i] = (i + 1) + ': ' + chunks[i];
    }
    return chunks.join('\n');
}
@param  {clay.Vector2} out
@param  {clay.Vector2} a
@param  {clay.Matrix4} m
@return {clay.Vector2}