Fix TileBoundingBox

[duvifn]

Fixes #4032.

See #4032 (comment).

This PR changes the way that the south plane is computed on the northern hemisphere (same for the north plane on the southern hemisphere).

Before:

After:

I didn't check if this also fixes other issues.

[pjcozzi]

Thanks for the detective work and fix, @duvifn, it is much appreciated!

@kring do you have any cycles to look at this?

[kring]

Looks good to me, thanks @duvifn!

[duvifn]

Thanks @pjcozzi and @kring for the quick treatment.
I really appreciate it!

[duvifn]

Just a little comment:
I added this fix to the top of CHANGES.md because I didn't know which version it would be shipped with.
Can you fix it in master?

[kring]

Yep, already did. Thanks for the reminder though, I did almost managed to forget about it!

[pjcozzi]

Thanks for the quick review, @kring. I will get to #5051 soon...this week, really.