GIMP should not convert assigned profile to sRGB just because we stored
as linear on the XCF. In other words, we should not look at the image
precision to decide whether to export as linear (previously only 8-bit
linear images), but at the profile TRC. There are basically 3 cases:
(1) We don't save a profile, then convert to sRGB whatever the source
precision (because readers would assume sRGB for a no-profile jpeg).
(2) We save the default profiles: convert to sRGB because it's usually
a better choice for 8-bit formats and even working at 32-bit float
*linear* doesn't mean you want to export as 8-bit int *linear*. As the
image creator made no explicit export choice, we make an acceptable
default one.
(3) We save an explicitly assigned profile: keep the profile TRC, don't
convert!

Note that this apparently won't work perfectly right now, as GIMP
replaces the original TRC with the linear default TRC when converting to
linear. So the expected TRC is lost in such case when you have not
explicitly reset the correct profile. Yet this is on GIMP side and this
part of the issue should be fixed with the space invasion merge. For
now, this is how the plug-in should work.

This is based on my late discussion with Ell. Please everyone, and Ell
especially, review! :-)

(cherry picked from commit c5f7bac2ba2121636bbf84d5e4af4d11e98fa511)