WebGLSnapshot = function (sourceContext, config)
{
    var gl = sourceContext;

    var callback = GetFastValue(config, 'callback');
    var type = GetFastValue(config, 'type', 'image/png');
    var encoderOptions = GetFastValue(config, 'encoder', 0.92);
    var x = Math.abs(Math.round(GetFastValue(config, 'x', 0)));
    var y = Math.abs(Math.round(GetFastValue(config, 'y', 0)));

    var getPixel = GetFastValue(config, 'getPixel', false);

    var isFramebuffer = GetFastValue(config, 'isFramebuffer', false);

    var bufferWidth = (isFramebuffer) ? GetFastValue(config, 'bufferWidth', 1) : gl.drawingBufferWidth;
    var bufferHeight = (isFramebuffer) ? GetFastValue(config, 'bufferHeight', 1) : gl.drawingBufferHeight;

    if (getPixel)
    {
        var pixel = new Uint8Array(4);

        var destY = (isFramebuffer) ? y : bufferHeight - y;

        gl.readPixels(x, destY, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);

        callback.call(null, new Color(pixel[0], pixel[1], pixel[2], pixel[3]));
    }
    else
    {
        var width = Math.floor(GetFastValue(config, 'width', bufferWidth));
        var height = Math.floor(GetFastValue(config, 'height', bufferHeight));

        var total = width * height * 4;

        var pixels = new Uint8Array(total);

        gl.readPixels(x, bufferHeight - y - height, width, height, gl.RGBA, gl.UNSIGNED_BYTE, pixels);

        var canvas = CanvasPool.createWebGL(this, width, height);
        var ctx = canvas.getContext('2d', { willReadFrequently: true });

        var imageData = ctx.getImageData(0, 0, width, height);

        var data = imageData.data;

        for (var py = 0; py < height; py++)
        {
            for (var px = 0; px < width; px++)
            {
                var sourceIndex = ((height - py - 1) * width + px) * 4;

                var destIndex = (isFramebuffer) ? total - ((py * width + (width - px)) * 4) : (py * width + px) * 4;

                data[destIndex + 0] = pixels[sourceIndex + 0];
                data[destIndex + 1] = pixels[sourceIndex + 1];
                data[destIndex + 2] = pixels[sourceIndex + 2];
                data[destIndex + 3] = pixels[sourceIndex + 3];
            }
        }

        ctx.putImageData(imageData, 0, 0);

        var image = new Image();

        image.onerror = function ()
        {
            callback.call(null);

            CanvasPool.remove(canvas);
        };

        image.onload = function ()
        {
            callback.call(null, image);

            CanvasPool.remove(canvas);
        };

        image.src = canvas.toDataURL(type, encoderOptions);
    }
}
Takes a snapshot of an area from the current frame displayed by a WebGL canvas.

This is then copied to an Image object. When this loads, the results are sent
to the callback provided in the Snapshot Configuration object.

@function Phaser.Renderer.Snapshot.WebGL
@since 3.0.0

@param {WebGLRenderingContext} sourceContext - The WebGL context to take a snapshot of.
@param {Phaser.Types.Renderer.Snapshot.SnapshotState} config - The snapshot configuration object.