The data offset value in the prefix increases as the heights increases.
This test adds a tests for some common dnxhr heights.

can somebody add the following test footage to fate/dnxhd?
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x1536.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x1716.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x2048.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x2160.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x3212.dnxhrSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>