Support mouseOut/pointerOut style event when the cursor leaves the game canvas

[eonarheim]

Context

It is often useful in games to know when the pointer leaves the game canvas. For example: pausing, deselecting, performing specific game logic.

Proposal

Create a new excalibur pointer event that tracks, perhaps analogous to the actor "exitviewport" and "enterviewport"

Current strategy

engine.canvas.onmouseout = () =>{
    console.log('Strike!')
};