Two small fixes

[KoffeinFlummi]

Fixes for two problems I encountered while integrating walkers into our application:

• By fullscreening the widget, violently moving the map around and also switching between tile providers, I was able to make the library panic by killing the IO thread. I assume this is happening because the channel for sending back the downloaded tiles is filling up. I fixed it by using the async version of send, and also downgrading the main thread's response from a panic to simply logging the death of the IO thread, since I don't want the entire application crashing because of that.
• The map was picking up scroll events when the pointer was outside of the map widget.

[podusowski]

Makes sense, thanks!