Fix pivot point in StreetParkingDrawable

[tapetis]

Closes #4590 (sorry).

Before my changes in 99be3a9, the car illustrations were shown correctly, but some cars were clipped when editing in a country with left-handed traffic:

After my changes in 99be3a9, the pivot point for the Canvas rotation / scaling was incorrect and the illustrations were not shown correctly:

With this PR both issues are fixed:

I tested my changes both with left-handed and right-handed traffic and did not notice any new issues.

[westnordost]

Noice!

So I understand there was a bug in the first place but it didn't show because the drawable always painted itself 2x as many times as necessary and you solved the root issue, thank you!

[westnordost]

Since the drawable is also shown in the street side select puzzle: Did you test if it shows correctly both in left-hand and right-hand driving countries there as well?

[tapetis]

Yes, the orientation dialog, position dialog, and the puzzle all looked good. I hope I didn't miss anything this time 🙈.