[opus] Potential transient pre-echo reduction filter

[Timothy B. Terriberry]

Aikku wrote:
> linear prediction. But since that would impact performance a fair bit 
> for my codec, I instead decided to take a lesson from wavelets and used 
> a simple sum/difference on every pair of coefficients (sort-of emulating 
> a Haar wavelet). ie. (excluding normalization factors)
> 
> a = X[i*2], b = X[i*2+1]
> X[i*2] = a + b
> X[i*2+1] = a - b

You might be interested in looking at RFC 6716 Section 4.3.4.5, and the 
corresponding source code in celt/bands.c's quant_band(), which calls 
haar1().