Multi-surface crash issue.

[credomane]

To reproduce:

1. have LTN Train Stops on multiple surfaces
2. Open LTN Tracker GUI
3. Click a LTN Train stop name in the LTN Tracker GUI that is NOT on the nauvis surface.
4. Factorio display a script error about expecting surface nauvis but entity is on "insert different surface name".

Looking though the code the issue seems to be a hardcoded value of "nauvis.

LTN_Tracker/script/gui_ctrl.lua

Lines 324 to 334 in 7e1213c

	local render_id = rendering.draw_circle({
	color = marker_color,
	radius = 3,
	width = 10,
	surface = "nauvis",
	filled = false,
	target = stop.entity,
	target_offset = {-1, -1},
	time_to_live = 300,
	players = {pind},
	})

Instead of surface = "nauvis" it should be probably be surface = stop.entity.surface

I don't know if that will fix the issue for certain but it has to at least be better than before right?