Render building labels from the same zoomlevel as landuse labels

Render building labels from 3000 pixels.

Old minimum pixel sizes for labels:
1643 on z14 (very rare), 3505 on z15 and z16.

This makes the code more consistent.
Merge pull request #1032 from math1985/building-label-minzoom

Render building labels from the same zoomlevel as landuse labels