Fix vehicle unseating on region crossings by ignoring outgoing sim kills

[Shadowolf7]

Description

This PR resolves the long-standing bug where avatars are forcibly unseated from vehicles during region crossings.

Root Cause

During a region crossing:

1. The avatar and vehicle cross into the receiving simulator, and the viewer sets its active agent region to the new simulator.
2. The outgoing simulator (lagging by 0–1s) cleans up its local physics representation of the vehicle and sends a UDP KillObject packet to the client.
3. Because the client did not differentiate between an active-region object deletion and an outgoing-simulator crossing cleanup, process_kill_object called markDead() on the vehicle prim.
4. In LLViewerObject::markDead(), lines 480-489 detected the seated avatar and executed ((LLVOAvatar*)childp)->getOffObject(), prematurely severing the sit link on the client side before the receiving simulator's ObjectUpdate took over.

Solution

In process_kill_object (indra/newview/llviewermessage.cpp), we check if a kill packet for the avatar's seated root object originates from a non-active (outgoing) region (regionp != gAgent.getRegion()). If so, the cleanup packet is ignored, keeping the vehicle and seated avatar intact across the boundary handoff.

Actual vehicle deletions (e.g. parcel auto-return, hard banlines, llDie()) sent by the active simulator (regionp == gAgent.getRegion()) continue to execute normally.

Verification

• Tested in-world across 16+ region crossings with multiple vehicles.
• Confirmed zero markDead() unseat ejections.
• Maintained continuous, uninterrupted seating through multiple consecutive border crossings.

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