Add a utility function to extract a location from the front of a vari…

…adic parameter pack

The purpose of this is to make it easy for functions implemented in Lua to handle locations
in the same way as functions implemented in C++.

Usage example:

Imagine a function foo with the following signature:

foo(bar : string, home : location, mode : string, target : location, moo : boolean) -> boolean

With the new read_location function it could be implemented as follows:

function foo(...)
	-- First argument goes in bar
	local bar = ...
	-- Read location starting at the second argument
	local home, n = wesnoth.mP.read_location(select(2, ...))
	-- note: n will be 0 if a location wasn't found at that position
	-- This could be an error, or it could be handled as an optional parameter
	-- Next argument after that goes in mode
	local mode = select(n + 2, ...)
	-- Then read a location into target
	local target, m = wesnoth.map.read_location(select(n + 2, ...))
	-- Finally, read a parameter into moo
	local moo = select(m + n + 2, ...)
	-- Do stuff with all these parameters
	return true
end

With that code, all the following invocations of foo work:

foo('a', 'b', true) -- both optional locations omitted
foo('a', 1, 2, 'q', 5, 6, false) -- locations given as separate integer parameters
foo('a', 'm', {1, 7},  true) -- first location omitted, second given as 2-element array
foo('a', some_unit, 'z', {x = 5, y = 10}, false) -- a unit also functions as a location
foo('a', 7, 12, 'q', my_leader, true) -- mixing different forms also works

data/lua/core.lua

@@ -681,6 +681,23 @@ if wesnoth.kernel_type() == "Game Lua Kernel" then
 			return code
 		end
 	end
+
+	function wesnoth.map.read_location(...)
+		local x, y = ...
+		if y == nil then
+			if type(x.x) == 'number' and type(x.y) == 'number' then
+				x, y = x.x, x.y
+			elseif type(x[1]) == 'number' and type(x[2]) == 'number' then
+				x, y = table.unpack(x)
+			else
+				return nil, 0
+			end
+			return {x = x, y = y}, 1
+		elseif type(x) == 'number' or type(y) == 'number' then
+			return {x = x, y = y}, 2
+		end
+		return nil, 0
+	end
 else
 	--[========[Backwards compatibility for wml.tovconfig]========]
 	local fake_vconfig_mt = {