:truck: New prop anim sources: `gearreverse`, `gearneutral`, `gear#`, `autoshifterlin`

[ohlidalp]

The gear* sources input 0 or 1 depending on the specific gear.

The autoshifterlin source works the same as shifterlin except it clamps forward gears to 1, so only has D/N/R positions. Fixes #2854

A test truck is attached - only the gear* flags are used though, I was lazy to test the shifter. propboard.zip