Fix #5225: Apply ped alpha to the jetpack clump

[x6c85]

Summary

CTaskSimpleJetPack::RenderJetPack draws the jetpack from its own clump, so setPedAlpha / me.alpha never reached it. The existing RenderJetPack hook now copies the ped clump alpha onto the jetpack materials, and skips render, thruster FX and engine sound when the ped is fully hidden (alpha 0 or a different interior).

Motivation

Fixes #5225. Getting a jetpack and running crun me.alpha = 0 hid the ped but left a floating pack.

Test plan

1. Give yourself a jetpack, then crun me.alpha = 0: ped, jetpack, thrusters and jetpack sound should all disappear.
2. crun me.alpha = 128: ped and jetpack should both be translucent.
3. crun me.alpha = 255: ped and jetpack should be fully visible again, sound restored.
4. Change interior while on a jetpack: pack should still hide/show with the ped as before.

Checklist

• Your code should follow the coding guidelines.
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