Fix a map exploration OOS on rejoin when starting with territory at the map boundaries.

rP9951 forgot to add a LosIsOffWorld check in ExploreTerritories (aka UpdateTerritoriesLos) and thus marked tiles outside of the world as explored.
rP13576 transformed the bug into a non-simulation desynchronization, causing rejoined players to see a different score,

as they excluded off-world tiles when filling the cache in ResetDerivedData upon rejoin.

rP19517 transformed the bug into an actual simulation OOS by serializing that map exploration percentage based on that cache.

Also tiles at the map border in square maps are not rendered as expected, so this commit hides refs #4267.

Differential Revision: https://code.wildfiregames.com/D630
Fixes #4598
Proofread by: Itms
Tested By: Imarok