gimp_display_shell_render() writes to a GeglBuffer backed by allocated memory
(shell->profile_data). Unfortunately while converting prevision in
gimp_image_convert_precision(), we change the "precision" property (hence the
source format) first, hence end up trying to write data in a too small buffer.
This crash was hard to find as it was not showing up on my machine (though it
did produce rendering artifacts!), unless I built both GIMP and babl with
`b_sanitize=address`.

Note that an alternate fix was to make sure that the profile_data buffer is big
enough (by calling gimp_display_shell_profile_update() before rendering), but
anyway the image is in an inconsistent state while conversion is in progress:
whereas the `src_format` is the new one, the `src_profile` is still the old one
(and cannot be changed before we finish converting).

Moreover the render happen regularly on progress signals, once after each
converted drawable. So each of these renderin...