Gif decoder: Provide optional alpha channel as an optional extra_channel, rather than using rgba pixels. (#1169)
Gif decoder: Provide optional alpha channel as an optional extra_channel, rather than using rgba pixels. (#1169)

* Modify gif.cc decoder to put alpha information into an optional extra_channel.

* gif.cc: Make gif decoder add alpha channel on all-or-none of the frames.
libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl,libjxl/libjxl