Split landcover and text-poly in high zoom (>= 10) and low zoom (< z10) layer

This causes significant speed improvements in z7, z8, and z9.
For example, rendering in z7 is now more than 5 times as fast.
Improve SQL layout
Add building IS NULL clause to low zoom landcover queries

This should improve performance
Merge branch 'low-zoom-performance' of https://github.com/math1985/openstreetmap-carto into math1985-low-zoom-performance

Conflicts:
	amenity-points.mss
Merge branch 'math1985-low-zoom-performance'