LibWeb: Don't relayout whenever a CSS transform changes

Transforms are a paint-level concept for us, so we should be okay to
only update the stacking context tree and repaint.

This makes a lot of CSS animations use way less CPU.

Userland/Libraries/LibWeb/CSS/Properties.json

@@ -1978,6 +1978,7 @@
   "transform": {
     "inherited": false,
     "initial": "none",
+    "affects-layout": false,
     "affects-stacking-context": true
   },
   "transform-origin": {