Add some quick-and-dirty debug info for TileStore (#2887)

* Add some quick-and-dirty debug info for TileStore

I'm still in need of more detailed data in order to understand why big layout shifts happen in large calls. This adds a developer option to enable logging and a visual indicator for the state of the TileStore. The indicator should be useful for matching up the behavior I'm seeing in my recordings with the right timestamps.

* Reduce performance impact of checking for whether debug mode is enabled

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Co-authored-by: Hugh Nimmo-Smith <hughns@element.io>

locales/en/app.json

@@ -70,6 +70,7 @@
   },
   "developer_mode": {
     "crypto_version": "Crypto version: {{version}}",
+    "debug_tile_layout_label": "Debug tile layout",
     "device_id": "Device ID: {{id}}",
     "duplicate_tiles_label": "Number of additional tile copies per participant",
     "hostname": "Hostname: {{hostname}}",