(fix) Avoid engine freeze when loading a new track while scratching with waveform or spinny

[ronso0]

Closes #15082

As proposed there, reset PositionScratchController on track load:

• release [ChannelN],scratch_position_enable
• reset the flags m_inertiaEnabled and m_isScratching
• reset m_rate

How to reproduce / confirm the fix:

• load a track
• left-click, move and hold its waveform or spinny to stop the track / start scratching
• load a track to this deck (Shift+Left or via controller)
• engine freeze (you click still do stuff in the GUI, eg. seek with in the overview but no sound anymore)
• closing Mixxx will close the main window but the process will not exit.

---

Waveform and spinny would still show the drag cursor (hand) but moving it would not scratch.
Though this minor inconvenience is negligible IMO.

[Swiftb0y]

Code LGTM. No testing done though.

[ronso0]

Thanks for taking a look!

I tweaked it a bit (avoid the potential double call in EngineBuffer::ejectTrack()).

And I added repro steps to the description.

[daschuer]

LGTM and the issue is fixed.