Fix indexing offset for backpack whilst metalpress container is current inventory context

[EngineersBox]

Changes

• Added offset to slot index when creating backpack slots

Edit: Was made aware that @Mi1ax had commented this fix on Ep8 so adding them as the original author of the fix.

[Mi1ax]

U just took my youtube comment and turned it into pull requests -.-

[EngineersBox]

Didn't realise you had left a comment. It's not exactly a unique fix though. I imagine it will be quickly outdated with further abstraction to a set of base machine classes.

Happy to add you as the original author of the fix though.

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On a side note, I would have included a more math oriented parallel version of inventory slot initialisation. I fear that it would be too out of context for the tutorial (people who are doing this for the very first time). It would also be pre-optimisation which can be quite dangerous.