scrollcontainer: Add automatic scrollbar calculation

[SmallJoker]

When specifying 'scroll factor'='auto', it will be calculated automatically based
on the associated scrollbar maximal value. Previously, calculating the exact value was mostly guesswork and try & error.

EDIT: This could also be implemented in reverse by adjusting the maximum value of the scrollbar, which would however be more annoying to read out from mods.

Part fix for #12482

To do

This PR is Ready for Review.

How to test

/c 0 (factor 0.005) vs /c 1 (automatic)

minetest.register_chatcommand("c", {
	func = function(name, param)
		local player = minetest.get_player_by_name(name)
		local value = minetest.is_yes(param) and "auto" or (5 / 1000)
		minetest.show_formspec(name, "lab:scrollcontainer",
			"formspec_version[7]"..
			"size[12,8]"..
			"scroll_container[0,0;10,4;scrbar2;vertical;" .. value .. "]"..
				"label[0.5,0.5;nest]"..
				"list[current_player;main;0,1;8,4;]"..
			"scroll_container_end[]"..
			"scrollbar[10.1,0;0.5,4;vertical;scrbar2;0]"..
			-- Scale
			"button[11,0;1,1;;0]"..
			"button[11,1;1,1;;1]"..
			"button[11,2;1,1;;2]"..
			"button[11,3;1,1;;3]"..
			"button[11,4;1,1;;4]"..
			""
		)
	end
})

[Desour]

Doesn't fix #12482 entirely, because that issue wants to set other scrollbaroptions as well, like thumb size.

[rubenwardy]

Aw that's what I thought this did...