Fix decoding GIF to 565

565 cannot take the !writeTransparentPixels path, so disable it for
cases where we might have to take that path.

This only affects frames beyond the first. If the first frame has
a transparent pixel, it will be marked as non-opaque, so we cannot
decode to 565 anyway.
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Review-Url: https://codereview.chromium.org/2441833002
4 files changed