Basic beach decorations

[Night-Pryanik]

Summary

Content "Basic beach decorations"

Purpose of change

Resurrect #70173 by @I-am-Erk.

Describe the solution

Big part of work was already done by @I-am-Erk, kudos to him, I just made the whole thing work and fixed some errors along the way.

Added most of the beach furniture and terrain from that PR:

• f_beach_log: an old log on the beach.
• f_broken_boat: a washed up chunk of an abandoned boat
• f_beach_seaweed: seaweed piled at the high waterline
• t_coast_rock_surf: a rocky coastline on the edge of the water, to connect to the surf and create a graphically interesting edge
• t_tidepool: a small basin of salt water in the sand.

Added the beach_formations connection group so that coast rock can connect to itself and its other versions, and sand can connect to it (but it doesn't connect to sand).

Instead of non-finished json version of map extra from that PR, I added a hardcoded version (for the time being). It doesn't create any "directional lines" of beach terrain or furniture, it just randomly populates the sandy beaches and surf line with various beach decorations.

Also, while I'm here, I made surf line look pretty instead of "straight cut" where it meets the sand.

Describe alternatives you've considered

Try to understand the idea of json mapgen of map extra from the PR and make it work.

Testing

Teleported to the ocean shore, hiked along the water, checked that sandy beaches are no longer empty.

Additional context

Before:

After:

Note: all sprites for the new terrain and furniture are placeholders.