Fix decoding tiff image with BigEndian + 64 bit / pixel + associated alpha

[Socolin]

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Description

This fix #3031 the method ColorFromRgba64Premultiplied seems to do the wrong things, I'm not 100% sure about the new code but it seems to works and kind of make sense. I'm trying to find another implementation as a reference to confirm this is the right things to do (But so far all the C library I checked are so hard to read I was not able to find confirmation...)