Color depths greater than 32 have grayscale bit 0x20 set, but the
decoder incorrectly treats 32 as grayscale and and clears the bit,
leaving the color depth as zero and causing codecs to fail.
Confirmed correct behavior in the ffmpeg code that the decoder was
based off. The decoder was introduced with the Mohawk engine in
2009,so presumably no Mohawk movies had color depth 32.
Fixes videos in the Director game Virtual Cocktail Bar
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