Let's create the folder returned by gimp_temp_directory() ourselves, the
first time this function is called, using a randomized name. This will
ensure that we won't conflict with another user running GIMP on the same
machine if using the base /tmp/ (which is usually the case on Linux).
Furthermore, since we create the temp folder at this level, we will also
delete it when quitting (but only if it's empty, as it should). Also I
use a single-level folder above the generic temporary directory, this
way, it's easier to track and we don't have to delete 2 levels of
directories anymore.
When the environment variable GIMP3_TEMPDIR is set though, we do not
create the folder, nor do we try and delete it.
This reverts commit 06cd254954.
GObject Introspection tools somehow fail to introspect some macros in a
very bizarre way. See issue #14668.
I thought #pragma once was somewhat standard, but it turns out it's not.
And Wikipedia even stays about this:
> While #pragma once is available in most modern compilers, its
> implementation is tricky and might not always be reliable.
Anyway clearly it's not reliable at least regarding GIR tools. I believe
we should hold up on using #pragma once at the very least within our
libgimp* libraries. It is probably fine for app/ or plug-ins/ where we
don't need to introspect anything (and so far we haven't had any
compilation mishap because of this preprocessing directive).
Since we consider it private, yet it's still needed in libgimp and app,
let's at least put it in a private header because there is no need for
people to try it out.
I'm also editing a bit the annotations for gimp_main() and GIMP_MAIN().
