Expose active tiles via API, sync by SGBucket.

Expose active tiles via API, sync by SGBucket.

Provide an API to query which tiles are currently being synced. This
allows the higher levels (TileManager in FlightGear) to safely wait on
TerraSync before loading tiles.

Doing this exposed some bugs in scheduling tiles to sync by integer
lat/lon, related to round-towards-zero with -ve values (western /
souther hemispheres). To resolve these, and also reduce spurious
syncs, switch to an explicit API for requesting tiles by SGBucket.
This keeps TerraSync fully aligned with the TileManager queue, which
has many benefits for both high and low visibility situations.