GIMP plugin alpha layer issue
Issue description
plugin will not properly display or save an image to jxl with an alpha layer GIMP 2.10.24
Environment
Operating System: linux (Absolute-current a Slackware-current derivative)
JPEG XL software version: git 20210509
Compilation:
-DCMAKE_INSTALL_PREFIX=/usr
-Dlibdir=/usr/lib64
-DCMAKE_BUILD_TYPE=Release -DJPEGXL_ENABLE_PLUGINS=ON -DJPEGXL_ENABLE_GIMP_SAVING=1 -DJPEGXL_ENABLE_DEVTOOLS=ON -DBUILD_TESTING=OFF -DJPEGXL_WARNINGS_AS_ERRORS=OFF -DJPEGXL_ENABLE_SJPEG=OFF ..
CPU / architecture / device: x86_64