Formspec: Fix clicking on tooltip-obstructed elements

[Desour]

• Goal of the PR: Fixing a bug.
• How does the PR work?
  irr::gui::IGUIElement::getElementFromPoint ignores invisible elements. Tooltips and inventorylists do not need to be visible to be drawn as they don't use the draw function.
• Fixes Formspec tooltip element prevents clicking on fields defined before it #9264.
• Side-note: This fix wouldn't work for other elements, and inventorylists should be done properly one day (with their own class instead of being drawn in GUIFormspecMenu). Other not-rectangular or holey elements need (best non-hacky) a way to pass not handled callbacks to elements below them.

To do

This PR is a Ready for Review.

How to test

local elements = [[
	list[current_player;main;0,0;4,4;]
	button[5,0;2,1;btn;Button]
	button[3,1.5;3,1;btn;Button2]
]]

local fs = [[
	size[11,10]
]] .. elements .. [[
	tooltip[0,0;10,5;<rect_mode tooltip defined after other elements>]
	container[0,6]
	tooltip[0,0;10,5;<rect_mode tooltip defined before other elements>]
]] .. elements .. [[
	container_end[]
]]

minetest.register_on_joinplayer(function(player)
	minetest.show_formspec(player:get_player_name(), "fs_test", fs)
end)

minetest.register_chatcommand("fs", {
	params = "",
	description = "Show the test formspec.",
	privs = {},
	func = function(name, param)
		minetest.show_formspec(name, "fs", fs)
		return true
	end,
})

[SmallJoker]

Works as expected