[Offline basemaps] Render tiles at lower zoom levels if available

[gino-m]

trafficstars

TileSetSource.boundsIntersect() only selects tile sets whose bounds intersect the viewport; tiles set which are strictly larger than the viewport are not rendered, even if available.

To get this to work we'll also need to find a way to render unavailable pixels as transparent, or we end up with large blank areas on the map:

@scolsen FYI