Improve JPEG output of 2-channel images #3531
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JFIF/JPEG files only support 1 and 3 channels. For > 3 channels, we had just been discarding the extra. For 2 channels, we had been discarding the second, but maybe that's not always right. Try to distinguish between files whose names appear to be YA -- in which case output the Y as grayscale (JFIF has no way to represent alpha anyway), and other cases which we now promote to RGB and just make B be black.
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JFIF/JPEG files only support 1 and 3 channels. For > 3 channels, we
had just been discarding the extra. For 2 channels, we had been
discarding the second, but maybe that's not always right. Try to
distinguish between files whose names appear to be YA -- in which case
output the Y as grayscale (JFIF has no way to represent alpha anyway),
and other cases which we now promote to RGB and just make B be black.