playerPlacementRiver function doesn't place teams properly in some cases

[Andy Alt]

playerPlacementRiver function doesn't place teams properly in some cases. For example, when setting up the random map Corinthian Isthmus, in a 7-player game, if the first 3 players are on Team 1, and the last 4 are on team 2, player 4 (counting top to bottom in the player list from the setup menu) will be on the side of the river where the 3 players on T1 are placed.

In a 5-player game, if the first 2 players are on T1 and the other 3 are on T2, player 3 will be placed with T1.

[marder]

In 27039:

[maps] Adjust playerPlacementRiver to group teams on the same side in most cases

Brute forcing the right arrangement, which is unnecessary but accepted since it has only a negligible performance impact overall.

accepted by: @lyv @andy5995

Fixes: #6390

Differential revision: ​https://code.wildfiregames.com/D4747