Auto merge of #15707 - froydnj:geckolib-transmute-removal, r=Wafflespeanut
Auto merge of #15707 - froydnj:geckolib-transmute-removal, r=Wafflespeanut

geckolib: remove std::mem::transmute usages

We can make these look nicer with the FFIArcHelpers trait.

I think there are tests for these changes?  Not sure.

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