[Feat] Add a viewport buffer around TileLayer

[jdgarrett]

Target Use Case

Right now if you pan or zoom a TileLayer, you'll see the new tiles come in on the edge of the screen, which leads to a choppy user experience. It would be nice if there was an option on the TileLayer to pre-load tiles off the edge of the screen so that no tile loading is visible in most cases (you'd probably still see tile loading when moving the camera quickly, such as with fly animations). This visual issue can be seen on the TileLayer example when you zoom in/out or pan the map.

Proposal

Maybe an extra option on the TileLayer for viewportBuffer or something where we can specify the number of pixels past the edge of the viewport to pre-load/render tiles in.