Orbit: bullets: call `_prepare_direction` before `_goto`

This is in line with other calls to `_goto`, which call
`_prepare_direction` before `_goto`.
See commit e577378ae6d772988bfef5f245069be664642fc7.

This fixes the animation when going to a previous slide.
Merge pull request #4952 from blueyed/orbit-bullets-prepare-direction

Orbit: bullets: call `_prepare_direction` before `_goto`