Bring back driving side switch for special purposes

[Kovoschiz]

Override doubles as workaround of previous #267

Sometimes people want to explain a layout using LHT examples in RHT settings to RHT countries or the majority of the rest of world

Therefore it can be useful to the public. A personal example, Taiwan is a road safety hell (several magnitude worse than USA, akin to an undeveloped country), and they love to cite Japan or Commonwealth examples by mirroring aerial imagery.

[dabreegster]

Done, you can change it at the bottom of the "Processing Options" on the left. The results are weird when there are one-ways, since those will still point the original direction: