Clean up definition tracks

The track definitions used to be rather inconsistent. The following
changes make the definitions more consistent across bridges, tunnels,
and normal roads.

- Give tracks without tracktype the same line-opacity as tracks with tracktype;
- Give grade3 tracks on bridges or in tunnels the same line-opacity as regular grade3 tracks;
- Give grade3 tracks on bridges or in tunnels dashes, just like regular grade3 tracks;
- Give grade2 tracks on bridges or in tunnels same dashes as regular grade2 tracks;
- Give grade3 tracks no wider white background in tunnels than on regular grade3 tracks.
Merge pull request #407 from math1985/track-rendering

Clean up definition tracks