Smart flat coordinates

[ahocevar]

This pull request simplifies geometry construction and improves performance by avoiding a change event upon construction.

After this change, geometry constructors accept flat coordinates, but no longer accept null as coordinates.

Replaces #8353.

[tonio]

LGTM, thanks @ahocevar for tackling this so quickly!
(unrelated, looks that I lost my commit rights at some time…)

[ahocevar]

Thanks for the review @tonio. I have restored your commit rights.

[tonio]

thanks!