Return shadow_sky_body_orbit_tilt setting

[x2048]

Used as a default value when the game does not change the value via API (e.g. legacy server)

Fixes #13302.

To do

This PR is Ready for Review.

How to test

1. Set the setting shadow_sky_body_orbit_tilt = 60
2. Enable shadows
3. Start a 5.7.0 game with shadows, notice the sun orbit tilt is 60 degrees
4. Connect to a 5.6.1 server, notice sun orbit tilt is 60 degrees
5. Add a mod with the following code:

minetest.register_on_joinplayer(function(player)
	player:set_sky({body_orbit_tilt = 30})
end)

7. Enable the mod and start the game
8. Notice the orbit tilt is 30 degrees

[x2048]

@Desour the problem was in not having a default value in SkyboxParams when deserialization fails of skips the value. I've returned the setting to act as a drop-in value in these cases.

[sfan5]

Are we sure we want to introduce a setting that only works on older server (this is mostly opaque to the user and will cause confusion)? Why not just hardcode 60 as default?

[x2048]

It will also work for games/servers that don't change orbit_tilt via API.

[lhofhansl]

Looks good (minus the warning)

[sfan5]

looks unused

[sfan5]

It will also work for games/servers that don't change orbit_tilt via API.

I see. Didn't review the code prior to commenting.
Will this work correctly with get_lighting() (so that it does not return -1024)?

[x2048]

I've added an if to avoid returning orbit_tilt from get_sky(true) when the value is 1024. It is not possible for the modders to set that value because of rangelim in l_set_sky

[sfan5]

should be documented in lua_api.txt that the field can be nil.

[x2048]

Expanded the documentation for the parameter, explaining the default behavior and changing the values.