fix orientation

[Davknapp]

Discovered a bug in the hypercube_pad example, where the orientation between two quad-faces was set to 4 (which is impossible if, a quad has 4 corners, we start with corner 0). This PR fixes it.

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[holke]

This feels like something that should have been caught by an assertion.

Looks like we are missing some checks?

[Davknapp]

This feels like something that should have been caught by an assertion.

Looks like we are missing some checks?

Yes, I found it while using this example in a test. Then an assertion was triggered. Checking if the orientation is correct is currently not possible during t8_cmesh_set_join, as we don't pass any information about the number of corners of the trees.
In my opinion the main cause of why this bug wasn't detected is that we didn't use the example in any tests.