Use g_file_open_tmp() instead of hand-rolling it

The glib-compile-resources tool has hand-rolled "open a temporary file"
code paths. Since error handling is hard, let's rely on GLib API that is
meant to do that consistently for us.

Get rid of some tabs mixed with spaces while we're at it.

https://bugzilla.gnome.org/show_bug.cgi?id=794284