Align:getPositionOffset(): add optional parameter to specify reference alignment

previously the offset was always relative to the coordinate origin (\an7 alignment),
now it's possible to use this method to calculate the offset between any given source and target alignment

Author: line0

l0/ASSFoundation/Tag/Align.lua

@@ -45,12 +45,15 @@ return function(ASS, ASSFInst, yutilsMissingMsg, createASSClass, re, util, unico
     function Align:isCenterH() return self:getSet("centerH") end
     function Align:isRight() return self:getSet("right") end
 
-    function Align:getPositionOffset(w, h)
+    function Align:getPositionOffset(w, h, refAlign)
+        refAlign = default(refAlign, Align{7})
         if ASS:instanceOf(w, ASS.Point, nil, true) then
             w, h = w:get()
         end
         local x, y = {w, 0, w/2}, {h, h/2, 0}
         local off = ASS.Point{x[self.value%3+1], y[math.ceil(self.value/3)]}
+        off:sub(x[refAlign.value%3+1], y[math.ceil(refAlign.value/3)])
+
         return off
     end