Fix TileSet thumb coordinate generation.
Fix TileSet thumb coordinate generation.

simonpoole/tilestream,markng/tilestream,HydroLogic/tilestream,frankrowe/tilestream,zhm/tilestream,mapbox/tilestream,HydroLogic/tilestream,frankrowe/tilestream,staltz/tilestream,staltz/tilestream,makinacorpus/tilestream,NolaMark/tilestream,NolaMark/tilestream,zhm/tilestream,markng/tilestream,mapbox/tilestream,makinacorpus/tilestream,simonpoole/tilestream