lavc/ffv1: Properly check that the 4th and 5th quant tables are zeroes

Currently, the decoder checks the 128th value of the 4th quant table during
while deriving the context on each sample, in order to speed itself up. This
is due to relying on the behavior of FFmpeg's FFV1 encoder, in which if that
value is zero, the entire 4th and 5th quant tables are assumed to be entirely
zero.

This does not match the FFV1 spec, which has no such restriction, and after
some discussion, it was decided to fix FFmpeg to abide by the spec, rather
than change the spec.

We will now check whether the 4th and 5th quant tables are zero properly,
by checking the 128th valye of both tables (which means they are zero due
to the way they're coded in the bitstream).

For further context, the FFV1 issue in question is located at:

    https://github.com/FFmpeg/FFV1/issues/169

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

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