resting on a sloped roof does not toss you off

## Testmerges
#10999: 'Fix of various plane related issues' by Blundir at commit f85a7153c4
#11414: 'Predship Multi-Z' by realforest2001 at commit 8a3037ed26
#11389: 'Yautja Leap' by realforest2001 at commit 0fd7966dd6
#11214: 'Proof of concept movielike pred invisibility' by Blundir at commit d8fc73ab73
#8689: 'AD-71E Blackfoot' by Git-Nivrak at commit fe210ab3f8

## Round ID
31939

## Description of the bug
title

## What's the difference with what should have happened?
should not be able to stay on a sloped roof

## How do we reproduce this bug?
1) pounce onto a sloped roof as xeno
2) rest

## Attached logs
```
N/A
```

## Additional details
- Author: Vending Machine
- Admin: Vending Machine
- Note: None

No source file or test is named. Reproduce the issue by having a xeno pounce onto a sloped roof and rest, then trace the movement and resting behavior involved; done means resting no longer allows the xeno to remain on the sloped roof.

Written by the indexing model from the issue text.