Fix encoder base+offset endpoint encodings #383
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For the RGB/RGBA base+offset endpoint formats the compressor can fail to detect cases that incorrectly do/do not trigger blue-contraction for the encoding path it is trying. This can result in endpoint encodings that are incorrect when decompressed because the blue-contraction the data was expecting is/is not applied.
This occurs because the compressor arithmetic is using shifted values for the implementation of
bit_transfer_signed()and follow on arithmetic, but doesn't factor in the shift when computing whether the endpoint R+G+B sum is invalid for the desired direction of blue-contraction. This only impacts the compressor; the decompressor is correct.Fixes #382