The javax.imageio JPEG plugin has code for supporting color spaces
which are optional according to the JPEG Plugin Metadata section
of the Image I/O specification.
Support for this is not present in OpenJDK and relies on
proprietary extensions to the IJG JPEG library.
One consequence is that the JPEG plugin in OpenJDK reports
it can write images with an alpha channel but in fact throws
an exception when asked to do so.
We should update or remove the code as appropriate