Road network data is represented as an undirected graph in a custom text format.

The format has two sections, vertices then edges, which together look like this:

    100 200
    100 400
    100 600
    200 400

    0 1
    1 0
    1 2
    2 1
    1 3
    3 1

The first four lines are vertices, while the last six are edges.
Each vertex line has two parts: the X coordinate and the Y coordinate.
These coordinates must be in the imagery coordinate system.

For example, the first vertex is at 100 pixels right and 200 pixels down from
the top-left corner of the corresponding image.

Each edge line has two parts: a source vertex and a destination vertex.
The vertices are referenced by their 0-indexed line number. So "0 1" connects
vertex (100, 200) to vertex (100, 400).

The format is capable of representing directed graphs. However, since road
networks in MUNO21 are undirected graphs, each edge line "a b" must have a
corresponding line "b a" elsewhere in the file.

The example graph above looks like this:

    (100, 200)
        |
    (100, 400) -- (200, 400)
        |
    (100, 600)