Scrolling veh_interact window

Changes the veh_interact window so that when you move, instead of moving the cursor, it moves the whole display; essentially keeping the cursor in the middle of the preview window.

This lets you now see every square of wide wreckage, and allows even bigger, more elaborate (or nonsensical) vehicles. In effect, you have unlimited construction space for vehicles now.

Fixes #1462
Fixes #2160