/** Number of the the next free data block; one data block consists of
64 short values in the data_blocks array.
- All blocks before this one are already claimed by filling their number
- into the corresponding blocks description structure field,
- that are hold in mv_blocks array.
+ All blocks before this one have already been claimed by placing their
+ position into the corresponding block description structure field,
+ that are part of the mv_blocks array.
- application - zeroes it on get_buffer().
A successful ff_draw_horiz_band() may zero it together
with start_mb_blocks_num.