Add formula= option to [set_variable] (change by @CelticMinstrel)

[ci skip]

changelog

@@ -66,6 +66,8 @@ Version 1.13.10+dev:
      improvement over the uncut ruby of fire (14x4) in TRoW.
    * Added [lua] as a conditional tag.
    * Lua errors now cause as a condition to fail instead of pass.
+   * New formula= key in [set_variable] allows evaluating a WFL formula with
+     the variable, which may be either a scalar or a container.
  * Miscellaneous and bug fixes:
    * Fixed crash after canceling add-on download (bug #2203)
    * Fixed ingame help showing units you haven't encountered (bug #2135)

data/lua/wml/set_variable.lua

@@ -98,6 +98,11 @@ function wesnoth.wml_actions.set_variable(cfg)
 		wesnoth.set_variable(name, helper.rand(tostring(cfg.rand)))
 	end
 
+	if cfg.formula then
+		local fcn = wesnoth.compile_formula(cfg.formula)
+		wesnoth.set_variable(name, fcn(wesnoth.get_variable(name)))
+	end
+
 	local join_child = helper.get_child(cfg, "join")
 	if join_child then
 		local array_name = join_child.variable or helper.wml_error "missing variable= attribute in [join]"