Recolor speed layer in pitched view when forward and backward speeds differ

[hlfan]

With #946, the view angle can now be used to filter speed signals. However, this filtering is not yet applied to tracks with differing forward and backward speed limits. Supporting those cases is more difficult, since unlike signals, tracks do not have a single inherent direction.

[hiddewie]

Good idea. This might be difficult to achieve, given that the railway lines are lines and not points. The direction (azimuth) of point features with directions, such as signals, are pre processed in the database to determine the azimuth. The line itself does not have a single azimuth. In particular looking sideways at a line that makes a tight turn would need to be "chopped" into pieces, each with a direction, based on the map bearing (camera direction).