lavc/vc1dec: fix a few "may be used uninitialized" warnings.
lavfi/idet: fix chroma subsampling with odd sizes.
lavfi/yadif: fix chroma subsampling with odd sizes.
lavf/img2enc: use FF_CEIL_RSHIFT().
avcodec/pthread: fix ffv1 with 2 threads

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>