hverkuil: you wanted to change the meaning of data_offset in past, but the doc still says "That data_offset is included in bytesused. " 
hverkuil: I was testing with svarbanov venus driver, the vp8 encoder needs that offset, and has issues, because data_offset was bigger then bytes used
which if I read the doc right, is a driver bug
I’ll test my code with vivid NV16 later on to make sure, and will add some bound checking, integer overflow isn’t nice ;-P