async queue for more responsive UI (needs more work)
async queue for more responsive UI (needs more work)

First approach in making the UI more responsive. Especially trying to
reduce panning being blocked while tiles are added - but this seems not
to suffice with larger tiles, thinking about web workers. Yet now each
tile is painted immediately. Still buggy, some tiles are left over when
panning fast.

nrenner/leaflet-tilelayer-vector,ebrelsford/leaflet-tilelayer-vector,ebrelsford/leaflet-tilelayer-vector