Improve pointer sync

[jtanadi]

Problem

When sending pointer info through ws, scotty currently sends pixel coordinates (window.clientX, window.clientY), but those values may be out of range for other clients.

Fix

Instead of sending absolute coordinates, send relative coordinates (like a percentage).

If one client sends something like:

{
  x: roundTo2(window.clientX / window.innerWidth),  // 0.42 
  y: roundTo2(window.clientY / window.innerHeight)  // 0.73
}

then other clients can consume that data like:

// example coordinates in px
const pointerX = window.innerWidth(mouseData.x)   // 853px
const pointerY = window.innerHeight(mouseData.y)  // 571px