WebGL Triangle + Texture

HTML

<!-- Vertex shader -->
<script id="shader-vs" type="x-shader/x-vertex">
    attribute vec4 vPosition;
    void main() {
        gl_Position = vPosition;
        gl_PointSize = 20.0;
    }
</script>

<!-- Fragment shader -->
<script id="shader-fs" type="x-shader/x-fragment">
    precision highp float;
    uniform sampler2D pTexture;
    void main() {
        vec4 pixel = texture2D(pTexture, gl_PointCoord);
        gl_FragColor = pixel;
    }
</script>

<!-- Canvas -->
<canvas id="canvas" width="500" height="500"></canvas>

CSS

html, body {
    background: #f2f2f2;
}
#canvas {
    background: #222;
}

JavaScript

var gl;
var shaderProgram;
var vertexBuffer;
var vertexAttrLoc;
var textureUniLoc;
var texture;

var canvas = document.getElementById('canvas');

// Initialises WebGL context (if available)
function initGL(canvas) {
    try {
        gl = canvas.getContext('experimental-webgl');
        gl.viewport(0, 0, canvas.width, canvas.height);
    } catch(error) {}
}

// Load and bind texture
function loadTexture() {
    texture = gl.createTexture(); // Returns allocated texture ID
    texture.image = new Image();
    texture.image.onLoad = function(){
        gl.bindTexture(gl.TEXTURE_2D, texture);
        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, texture.image);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
        gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
    };
    texture.image.src = 'http://images.wikia.com/hogwartsrpg/images/9/95/KittenCareCat.png';
}

// Loads and compiles shader code from the DOM
function loadShader(id) {
    var src = document.getElementById(id);
    
    if (!src) { return null; }
 
    var str = "";
    var k = src.firstChild;
    while (k) {
      if (k.nodeType == 3) {
        str += k.textContent;
      }
      k = k.nextSibling;
    }
 
    var shader;
    if (src.type == "x-shader/x-fragment") {
      shader = gl.createShader(gl.FRAGMENT_SHADER);
    } else if (src.type == "x-shader/x-vertex") {
      shader = gl.createShader(gl.VERTEX_SHADER);
    } else {
      return null;
    }
 
    gl.shaderSource(shader, str);
    gl.compileShader(shader);
 
    if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
      alert(gl.getShaderInfoLog(shader));
      return null;
    }
 
    return shader;
}

// Initialises shaders and creates the shader program
function initShaders() {
    var vertShader =...