ND2: only adjust for {Frame,Time}{Before,After} if LoopSize is used

Fixes #11997.
ND2: do not skip bytes at the end of text info blocks

Fixes #11873.
Merge pull request #1178 from melissalinkert/rebased/develop/nd2-part-2

Nikon .nd2 fixes, part 2 (rebased onto develop)