Don't hammer on the layout engine with avatar updates for the background

Changing the property on every render of the left panel (which is basically all the time) is super bad on the GPU and for our CPU. We should only do that when something changes.

src/settings/Settings.js

@@ -166,6 +166,10 @@ export const SETTINGS = {
         displayName: _td("Show info about bridges in room settings"),
         default: false,
     },
+    "RoomList.backgroundImage": {
+        supportedLevels: LEVELS_ACCOUNT_SETTINGS,
+        default: null,
+    },
     "baseFontSize": {
         displayName: _td("Font size"),
         supportedLevels: LEVELS_ACCOUNT_SETTINGS,