The bag and written book currently don't listen to hand roll at all. We should try to get that to work.

If I ever revisit this, stack.mulPose(Axis.ZP.rotation((float) Math.PI + info.handRoll)); works for ImmersiveBackpack#render() to render the bag model correctly. Couldn't figure out how to get the hitbox positioning to work, though.

EDIT: For future self, maybe redo how positions are done with the bag, by rotating a point similar to how point in OBB detection is done.

Using the new OBB system combined with the knowledge I gained from working on #399, this is much more doable now! Should be done for both the bag and the written book immersive.

Book has been done for 1.20.4 here: bbd2603. Will do bag for 1.20.4, then downport both through the rest of the Minecraft versions.

Bag is done for 1.20.4 here: 739ccdc. Need to downport this through the rest of the versions, then this will be done!

Totally forgot to check left-handedness for the bag. Once I'm done downporting, I'll check that, make any fixes if needed, and downport those.

Downported! I did do left-handedness correctly, so no need to double back and fix things up; nothing is broken! This is now finally done.