av1: use chroma txtp inference over default DCT_DCT if qidx=0
Fixes #320. The problem here is that qidx=0 is (in libaom) a shortcut for lossless, which normally becomes WHT_WHT, but under some obscure conditions, it can also be non-lossless, in which case qidx=0 implies DCT_DCT for luma. For chroma, apparently we should use the default inference pattern, which becomes DCT_DCT for inter also, but requires the standard lookup table for intra.
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