Migrate GLR to metarules

[memo33]

'tis a beaut.

This replaces the old draggable GLR code by a rock-solid purely metarules-based implementation.

This PR depends on #557.

The old handwritten GLR code had some flaws:

• The orthogonal piece often switched orientation in the direction the network was dragged, which can lead to instability.
• The GLR code hadn't been kept in sync with changes to the Lightrail rules (such as the removal of the traffic circle from a four-way intersection).

---

Ideas for future extensions:

• add IDs for end stubs
• add support for Lightrail × GLR

These would require some work on textures/paths/models.