function getMousePos(canvas, evt) {
  var rect = canvas.getBoundingClientRect();
  return {
    x: evt.clientX - rect.left,
    y: evt.clientY - rect.top
  };
}
The mouseMoved() function is called every time the mouse moves and a mouse
button is not pressed.<br><br>
Browsers may have different default
behaviors attached to various mouse events. To prevent any default
behavior for this event, add `return false` to the end of the method.

@method mouseMoved
@example
<div>
<code>
// Move the mouse across the page
// to change its value

var value = 0;
function draw() {
  fill(value);
  rect(25, 25, 50, 50);
}
function mouseMoved() {
  value = value + 5;
  if (value > 255) {
    value = 0;
  }
}
</code>
</div>

<div class="norender">
<code>
function mouseMoved() {
  ellipse(mouseX, mouseY, 5, 5);
  // prevent default
  return false;
}
</code>
</div>