如何使用WebGL允许2D形状环绕画布？

<canvas id="myCanvas"></canvas>

var canvas = document.getElementById("myCanvas");
var gl = canvas.getContext("webgl");

gl.viewport(0, 0, canvas.width, canvas.height);

// 创建顶点着色器
var vertexShaderSource = `
    attribute vec2 a_position;
    void main() {
        gl_Position = vec4(a_position, 0, 1);
    }
`;
var vertexShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vertexShader, vertexShaderSource);
gl.compileShader(vertexShader);

// 创建片段着色器
var fragmentShaderSource = `
    void main() {
        gl_FragColor = vec4(1, 0, 0, 1);
    }
`;
var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fragmentShader, fragmentShaderSource);
gl.compileShader(fragmentShader);

var program = gl.createProgram();
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);
gl.linkProgram(program);
gl.useProgram(program);

var positionBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
var positions = [
    -0.5, -0.5,
    0.5, -0.5,
    0.5, 0.5,
    -0.5, 0.5
];
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

var positionAttributeLocation = gl.getAttribLocation(program, "a_position");
gl.enableVertexAttribArray(positionAttributeLocation);
gl.vertexAttribPointer(positionAttributeLocation, 2, gl.FLOAT, false, 0, 0);

gl.drawArrays(gl.TRIANGLE_FAN, 0, 4);