On iOS, don't attempt to write the tile dump ppm file to /tmp, but use
the app-specific directory so that it can be copied for inspection
using iTunes. (Of course, even better would be to simply paint it to
the app's view. Later)

Change-Id: I8dd60d04adc61de6594099f5c358a9b6220522da
Reviewed-on: https://gerrit.libreoffice.org/61255Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>