Allow scaling on mobile

[bozdoz]

Might be as simple as adjusting the viewport meta tag in index.html:

<meta name="viewport" content="user-scalable=no, initial-scale=1, maximum-scale=1, minimum-scale=1, width=device-width" />

[bozdoz]

Isn't. :(

[bozdoz]

Follow this method: https://stackoverflow.com/a/3151987/488784

    // Get mouse offset.
    var mousex = event.clientX - canvas.offsetLeft;
    var mousey = event.clientY - canvas.offsetTop;
    // Normalize wheel to +1 or -1.
    var wheel = event.deltaY < 0 ? 1 : -1;

    // Compute zoom factor.
    var zoom = Math.exp(wheel*zoomIntensity);

    // Translate so the visible origin is at the context's origin.
    context.translate(originx, originy);

    // Compute the new visible origin. Originally the mouse is at a
    // distance mouse/scale from the corner, we want the point under
    // the mouse to remain in the same place after the zoom, but this
    // is at mouse/new_scale away from the corner. Therefore we need to
    // shift the origin (coordinates of the corner) to account for this.
    originx -= mousex/(scale*zoom) - mousex/scale;
    originy -= mousey/(scale*zoom) - mousey/scale;

    // Scale it (centered around the origin due to the trasnslate above).
    context.scale(zoom, zoom);
    // Offset the visible origin to it's proper position.
    context.translate(-originx, -originy);

    // Update scale and others.
    scale *= zoom;
    visibleWidth = width / scale;
    visibleHeight = height / scale;