setBlendFunc() {
        // allows transparency
        // based on how three.js solves this
        this.gl.enable(this.gl.BLEND);
        if(this.premultipliedAlpha) {
            this.gl.blendFuncSeparate(this.gl.ONE, this.gl.ONE_MINUS_SRC_ALPHA, this.gl.ONE, this.gl.ONE_MINUS_SRC_ALPHA);
        }
        else {
            this.gl.blendFuncSeparate(this.gl.SRC_ALPHA, this.gl.ONE_MINUS_SRC_ALPHA, this.gl.ONE, this.gl.ONE_MINUS_SRC_ALPHA);
        }
    }
Called to set the blending function (transparency)
*