Load tiles under cursor first #21
Description
It would feel a lot snappier to download whatever is at the center of the zoom first (that's usually where I am looking and I assume others too). We can get axis mouse position and use it to define a square that gets queued first for each zoom level.
Currently the download is random, and after #13 its top to bottom, so the first tiles may even be mostly off screen.
This follows on from #13