fix: fixes cabin pool having salt water shallows

[chaosvolt]

Purpose of change

Found that the lil pool around one cabin variant had normal water for its deep water, but salt water for the shallows.

Describe the solution

Set the cabin palette to use regular water for shallow water, like it does for deep water.

Describe alternatives you've considered

Doing it the other way around because having a giant pool of salt water outside your house with a water slide is definitely logical. :3

Testing

1. Checked that this cabin variant was the only actual case where q and Q from the palette actually show up, and that No Hope didn't do anything screwy with mapgen palettes for once.
2. Checked that t_water_dp is indeed the correct terrain ID.

Additional context

Checklist