console: modify ppm_save to take a pixman image ref

The function is going to be called from a coroutine, and may yield.
Let's ensure our image reference doesn't change over time (due to resize
etc) by keeping a ref.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20201027133602.3038018-3-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>