Release shadow mapping resources when not needed

[x2048]

This PR removes the shadow mapping textures to release GPU memory when shadow intensity is changed to 0. It also cleans up code where shadow texture is assigned to map and entities.

To do

This PR is Ready for Review.

How to test

1. Enable the following mod:

core.register_chatcommand("s", {
	params = "intensity",
	func = function(name, param)
		local player = core.get_player_by_name(name)
		player:set_lighting({ shadows = { intensity = tonumber(param) } })
	end
})

2. Enable the shadows and enter the game
3. Toggle shadows with /s 0 and /s 1 commands
4. Shadows must disappear and reappear without error messages or crashing the game

[lhofhansl]

Looks like this is breaking self-shadowing of entities.
You can try with the https://content.minetest.net/packages/apercy/steampunk_blimp/ blimp. Notice that with this change the airship does not a shadow on itself - or more precisely it always seems to be in the shade.

[lhofhansl]

Works.