Fix for triangular bounding boxes

[vhf]

Here's a fix for one of the two bugs described in #4 .

I haven't yet found a fix for zero area bounding boxes.

[Jdyn]

Any chance of merging this? I have been encountering this as well.

I have also encountered the crash from inserting zero area bounding boxes as well, but it was by error. That is, I didn't actually mean to insert zero area, but for example if the input is user generated, it was possible. Of course you could create a check before feeding it to insert but I wonder if the library could take care of that check? Is there any use case to insert a zero area box besides by accident?

@vhf do you still use this library? I noticed that if you call functions like insert and the key is already there, it still returns :ok even though the operation was not successful. Say if the key was already in the tree, you wanted to update it instead, a.k.a upsert, it doesn't seem straight foward to do since it always returns :ok. Do you think updating the library to return responses like {:already_exists, nil} will improve the library?

[vhf]

@vhf do you still use this library?

I don't. I can confirm this PR works though, I used it for a couple of years. The studio behind this lib hasn't been active the past 3 years, if I still needed this lib I'd try to get access to the repo and package from the original maintainers.

[sikanrong]

@Jdyn this project is unmaintained but I merged this PR. Didn't test it but the code makes sense / seems fine to me