Pins disappear from GTK MapView after scroll or update

Describe the bug

When using a MapView under GTK, pins will sometimes disappear from the map if their location is updated, or the map is scrolled while the pin is visible on the screen.

Steps to reproduce

1. Run the mapview example
2. Click on the (3) button to move to the location of Carmen Sandiego
3. Click on the (Move) button to move Carmen
4. Click on the (3) button again

You'll move to a new location, but the Carmen Sandiego pin won't be visible

It may be necessary to do steps 3 and 4 a couple of times for the problem to manifest.

Expected behavior

The Carmen Sandiego pin should be visible at the new location.

Screenshots

No response

Environment

• Operating System: Ubuntu 22.04
• Python version: 3.10
• Software versions:
  • Briefcase: 0.3.17
  • Toga: #2379

Logs

No response

Additional context

This is a known issue with Leaflet.js, which is the underlying implementation of MapView on GTK. If/when that issue is addressed, it should be possible to fix Toga by updating the version of leaflet that is pinned in the GTK backend's MAPVIEW_HTML_CONTENT.

Check Leaflet.js issue 9220 first, since the GTK MapView behavior is identified as an upstream problem. Then inspect the GTK backend's pinned version in MAPVIEW_HTML_CONTENT and reproduce the issue with the mapview example using the listed Carmen Sandiego steps. Done means the pin remains visible after moving and scrolling once an appropriate Leaflet update is available.