Rationalize road sizes

These changes make sure that more important roads are not rendered
smaller than less important roads.

- Render motorways from z12 on wider.
- Render trunk, primary, secondary, and tertiary on z13 and z14 narrower.
- Render casing on tertiary link wider.

This solves #256.
Merge branch 'road-widths-2' of https://github.com/math1985/openstreetmap-carto into math1985-road-widths-2

Conflicts:
	roads.mss
Merge branch 'math1985-road-widths-2'