[variants] Make it possible to raise the oxygen when there's no space for greeneries

[kberg]

A discussion which starts at https://discord.com/channels/737945098695999559/738006783955697704/932736575610175520 covers how once you tread into some of the fan expansions, it's possible, though probably rare, to find it impossible to add greeneries, regardless of how much money you have, thereby preventing a game from coming to a conclusion.

One workaround is to create a new standard project that is only available when a player can afford a greenery, but cannot place one: spend 8 plants or 13MC to raise oxygen 1%

[kberg]

1. Pathfinders contributes to this problem.

2. The Political Agendas expansion has a Reds policy: "Pay 4 M€ to reduce a non-maxed global parameter 1 step"

3. It's possible on Arabia Terra for a player to put oceans on the cove spaces, making less space available for greeneries.

[kberg]

Some of the discussion on the thread also pointed out how unlikely this is to occur.

[kberg]

There might be a similar need for Moon standard projects, which require placing tiles.