Fix some Game members not being freed after some startup errors (regression)

[grorp]

This PR fixes a regression from fd8e021. In that commit, I moved some freeing from Game::~Game to Game::shutdown. Now I realize that Game::shutdown isn't called in all cases, for example when a shader compilation error occurs.

Behavior in case of a shader compilation error:

Version	Behavior
master with fd8e021 reverted	Loading screen freezes for 5 seconds in "Initializing nodes..." phase while server shuts down. Shader error is shown afterwards.
master	Shader error is shown immediately. Server is not shut down and can be joined using a second client.
this PR	"Shutting down..." screen is shown for 5 seconds without freezing while server shuts down. Shader error is shown afterwards.

This PR works by always calling Game::shutdown. Somewhat related old PR: #9742

To do

This PR is a Ready for Review.

Notes:

• I think the stuff I moved from Game::~Game to Game::shutdown could actually be moved back. However, that would mean spawning a thread (StopThread) and rendering the shutdown screen in a destructor, which feels bad. What if one of these things throws an exception? Would this be a better solution?

• Could always calling Game::shutdown introduce new bugs?

How to test

Edit shaders to make shader compilation fail. Add this code to builtin/game/init.lua:

minetest.register_on_shutdown(function()
	print("wa wa wa waiting")
	local t = os.time() + 5
	while os.time() < t do
	end
	print("finished waiting")
end)

Verify that the behavior of different versions matches what I describe in the table above.

[sfan5]

I think a C++-y approach would be if ~Game() contained code to destroy all members.
shutdown() is just then the normal way to deinit the client, but in exceptional circumstances still nothing would leak.

[grorp]

When I first implemented your suggestion, it unfortunately made things worse: When a shader error occured, the loading screen froze again during server shutdown. To fix this, I moved some exception catching down from ClientLauncher to Game. I hope this doesn't have unexpected side effects...

[sfan5]

Looks good.