In order to implement the min attribute properly we need to distinguish between
cases where the repeat duration is less than the active duration so we can apply
the fill mode in the 'gap'. Currently the repeat duration calculation is mostly
contained in GetRepeatDuration (but is hard to make sense of) and partly
contained in the call site CalcActiveEnd. Furthermore, it does not have thorough
tests (there were some unit tests but they were never converted to mochitests).
This patch reworks the repeat duration calculation so it is contained in one
place, easier to read, and thoroughly tested.
