[copybara-service]

Let jxl_bridge callers decode extra channels.

jxl_bridge discards every extra channel except an alpha or black channel
that the channel layout interleaves with the color channels. This adds
an opt-in way to get all of them:

decoder.SetPixelLayoutWithExtraChannels(layout, sample_format);
decoder.DecodeFrame(data, color, extra_channels);

extra_channels receives one width * height plane per extra channel,
back to back in the order of BasicInfo::extra_channels, in the same
sample format as the color channels. Existing calls are unaffected:
SetPixelLayout() still ignores extra channels.

This is for images whose extra channels carry data rather than
transparency, such as the bands of a multispectral image beyond the
first one or three.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.