fix #611; add distance check when mounting pet

Fix issue #611 (closed): Fix mountable pets.

This merge requests adds a distance check to the mounting procedure - ignoring all entities that are too far away. To mount a pet after merging this, the distance between player and pet must be below 20000.

The player is allowed to be mid-air when performing a mount. In my opinion, this gives the game a more dynamic feel.

I wasn't able to find a simple solution based on the previous iterator implementation, so I unrolled the ecs join into a loop.

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• Significant changes of this merge request have been added to the changelog.