setFragmentDiscardExpression(expression) {
    this.discardExpression_ = expression;
    return this;
  }
Sets an expression to determine whether a fragment (pixel) should be discarded,
i.e. not drawn at all.
This expression can use all the uniforms, varyings and attributes available
in the fragment shader, and should evaluate to a `bool` value (it will be
used in an `if` statement)
@param {string} expression Fragment discard expression
@return {ShaderBuilder} the builder object