Bugfix: Timer.update(dt) sometimes fails on `for'

Deleting functions from the function register while looping over
it could break the next-iterator in the for loop. To prevent that
from happening, we collect the functions to be deleted and delete
them in an extra loop.
Functions will still only be called after they have been removed
from the timer register.

timer.lua

@@ -27,15 +27,19 @@ THE SOFTWARE.
 Timer = {}
 Timer.functions = {}
 function Timer.update(dt)
+	local to_remove = {}
 	for func, delay in pairs(Timer.functions) do
 		delay = delay - dt
 		if delay <= 0 then
-			Timer.functions[func] = nil
-			func(func)
+			to_remove[#to_remove+1] = func
 		else
 			Timer.functions[func] = delay
 		end
 	end
+	for _,func in ipairs(to_remove) do
+		Timer.functions[func] = nil
+		func(func)
+	end
 end
 
 function Timer.add(delay, func)