Listen for movement on the document, not the canvas

This way, parts of the side of the image can be revealed.

xray.js

@@ -55,13 +55,13 @@ document.addEventListener("DOMContentLoaded", e => {
 
     const overlay = document.querySelector("#overlay");
     
-    overlay.addEventListener("mousemove", e => {
+    document.addEventListener("mousemove", e => {
         let x = e.clientX - e.target.getBoundingClientRect().x;
         let y = e.clientY - e.target.getBoundingClientRect().y;
         updateOverlay([[x,y]]);
     });
 
-    overlay.addEventListener("touchmove", e => {
+    document.addEventListener("touchmove", e => {
         let offsetX = e.target.getBoundingClientRect().x;
         let offsetY = e.target.getBoundingClientRect().y;