Henrik Gramner authored Jul 18, 2023 and Henrik Gramner committed Jul 18, 2023

Reduces memory usage (by 3 kB per sb128 for 4:2:0) when decoding
streams with subsampled chroma when frame threading is enabled.

This also simplifies the logic for calculating cbi indices.
Both entropy decoding and reconstruction access the elements in
the same order, so calculating block x/y positions is redundant
and we can instead just store values sequentially and increase
the pointer by one every time it's accessed.