Change assertion in fade tag to check durations rather than time values

Old behavior was to check if the 2nd and 3rd stops were positive, which makes no sense as negative time values are allowed in fade. Negative deltas, however, are not.

Author: CoffeeFlux

l0/ASSFoundation/Tag/Fade.lua

@@ -30,10 +30,11 @@ return function(ASS, ASSFInst, yutilsMissingMsg, createASSClass, re, util, unico
             return self.startDuration:getTagParams(coerce), self.endDuration:getTagParams(coerce)
         else
             local t1, t4 = self.startTime:getTagParams(coerce), self.endTime:getTagParams(coerce)
-            local t2 = t1 + self.startDuration:getTagParams(coerce)
-            local t3 = t4 - self.endDuration:getTagParams(coerce)
+            local startDuration, endDuration = self.startDuration:getTagParams(coerce), self.endDuration:getTagParams(coerce)
+            local t2 = t1 + startDuration
+            local t3 = t4 - endDuration
             if not coerce then
-                 self:checkPositive(t2,t3)
+                 self:checkPositive(startDuration,endDuration)
                  assertEx(t1<=t2 and t2<=t3 and t3<=t4, "fade times must evaluate to t1<=t2<=t3<=t4, got %d<=%d<=%d<=%d.",
                           t1,t2,t3,t4)
             end
@@ -52,4 +53,4 @@ return function(ASS, ASSFInst, yutilsMissingMsg, createASSClass, re, util, unico
     end
 
     return Fade
-end
+end