dpx: Fix rounding in 10bit total_size calculation

This could have caused out of array reads

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
dpx: include offset in the  total_size calculation

Fixes out of array reads

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>