Merge #tracks-notunnel-nobridge into #roads-fill

The layer #tracks-notunnel-nobridge has been merged into #roads-fill.

This unifies the various track-definitions.

Rendering changes:
The rendering order of tracks versus other roads is now determined by z_order.
Therefore, tracks are now rendered at the same level as service roads, footways, cycleways, and paths, instead of below them.
The rendering of the tracks themselves has not been altered.