prevent erroneous edit of wrong parcel

[AndrewMeadows]

This PR fixes jira-archive-internal/issues/70771

https://github.com/secondlife/jira-archive-internal/issues/70771

The problem was the update for the selected parcel was being sent to the wrong region, because the viewer logic for finding the corresponding region was flawed.

[AndrewMeadows]

This is where the bug lived. The region pointer was being found based on (a) whether the agent was in the region or (b) a cached value of mWestSouth which was supposed to be the global-position of the region's origin corner, but could sometimes be from the wrong region.

The fix is to cache the region_id in each parcel so it can be used to find the region in this context.

[AndrewMeadows]

I think the repro recipe goes something like this:

(1) Find two adjacent regions A and B, each with only one parcel (to make things easy)
(2) While on regionA sidle up to the boundary with regionB, but don't cross over.
(3) Select parcelA on regionA and open About Land floater (just to make it visible)
(4) On regionA terrain right-click the terrain and select Edit Terrain
(5) Left-click parcelB on regionB --> notice the parcel info populates the previously openened About Land floater
(6) Change the Name of parcelB
(7) Select parcelA in regionA --> notice its name has changed to what you tried to change on parcelB

Why does this work? Because there is a way to cache the LLViewerParcelMgr::mWestSouth corner of regionA and then change to a parcel in regionB without updating that cached value. When the old viewer would try to apply a parcel properties change it would use the cached value of mWestSouth to figure out which region to send it to. I'm not 100% sure that is the exact repro recipe above, but it is something like that. The operation of opening the About Land floater up will, for most cases, correctly cache mWestSouth for the appropriate region, so you need to find that path that caches the corner and then later updates the About Land parcel selection without recomputing it.