Work around Face3::get_area() bug in engine

[jonri]

We rely on the Face3::get_area() function returning the correct result, but it returns a value twice the actual area.

Until the pull request with the fix is merged, we could divide the result of the function in half ourselves. This should also be noted in the documentation in case someone fixes the function on their own.

godotengine/godot#37048
godotengine/godot#37064

[GNSS-Stylist]

For a case someone stumbles upon buoyancy-related issues: Pull request mentioned has been merged to godot's master and 3.x-branches. So if someone is using those they may want to remove the workaround.

[fire]

@jonri

[jonri]

Thanks. Since this fix will appear in 3.4, I can add some automatic version detection to apply/remove the workaround at compile time.

[jonri]

Unless a user has manually fixed the bug in the engine source, the version detection added in d5c34d6 should just do the right thing without any user intervention.