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Jean-Marc's original anti-collapse patch used a threshold on the content of a decoded band to determine whether or not it should be filled with random noise. Since this is highly sensitive to the accuracy of the implementation, it could lead to significant decoder output differences even if decoding error up to that point was relatively small. This patch detects collapsed bands from the output of the vector quantizer, using exact integer arithmetic. It makes two simplifying assumptions: a) If either input to haar1() is non-zero during TF resolution adjustments, then the output will be non-zero. b) If the content of a block is non-zero in any of the bands that are used for folding, then the folded output will be non-zero. b) in particular is likely to be false when SPREAD_NONE is used. It also ignores the case where mid and side are orthogonal in stereo_merge, but this is relatively unlikely. This misses just over 3% of the cases that Jean-Marc's anti-collapse detection strategy would catch, but does not mis-classify any (all detected collapses are true collapses). This patch overloads the "fill" parameter to mark which blocks have non-zero content for folding. As a consequence, if a set of blocks on one side of a split has collapsed, _no_ folding is done: the result would be zero anyway, except for short blocks with SPREAD_AGGRESSIVE that are split down to a single block, but a) that means a lot of bits were available so a collapse is unlikely and b) anti-collapse can fill the block anyway, if it's used. This also means that if itheta==0 or itheta==16384, we no longer fold at all on that side (even with long blocks), since we'd be multiplying the result by zero anyway.21af73eb
