Fade in opacity of tile plots as they load

[rafaqz]

This might be overkill...

But google maps seems to avoid flickery tiles by loading more tiles at once. Or something. Tyler seems faster at getting tiles loaded than google maps, which is kinda surprising, but it gives this flickery effect.

If we fade in tile opacity for e.g. the first 0.2 seconds we could remove the tile flicker? Maybe that's expensive?