DataMaps should only load visible tile, and lazyload tiles that are outside of view

[alistair3149]

Describe the bug
DataMaps is loading all image tiles even if they are outside of the map viewport. This is causing a lot of unnessecary network load and it defeats the benefit of having a tile map.

To Reproduce
Steps to reproduce the behavior:

1. Create a tile map
2. Set your map to zoom into a marker, or use the mini layout
3. Observe the network request in your browser devtools

Expected behavior
Only images that are visible in the map view are loaded

Live demo
https://tolkiengateway.net/wiki/User:Alistair3149/sandbox/datamapmini