fix: treat varargs for min and max in luamath

Fixes pgf-tikz/pgfplots#492

Signed-off-by: Henri Menke <henri@henrimenke.de>

Author: hmenke

tex/generic/pgf/libraries/luamath/pgf/luamath/functions.lua

@@ -148,12 +148,12 @@ function pgfluamathfunctions.less(x,y)
   end
 end
 
-function pgfluamathfunctions.min(x,y)
-  return mathmin(x,y)
+function pgfluamathfunctions.min(x,y,...)
+  return mathmin(x,y,...)
 end
 
-function pgfluamathfunctions.max(x,y)
-  return mathmax(x,y)
+function pgfluamathfunctions.max(x,y,...)
+  return mathmax(x,y,...)
 end
 
 function pgfluamathfunctions.notequal(x,y)