shared: Add a function to prefix filenames with datadir

shared: Add a function to prefix filenames with datadir

Currently we look for png files in their install directory, and we
manufacture filenames with string pasting at compile time.

This new function will allow overriding the compile time setting with
the env var WESTON_DATA_DIR so we can do neat tricks like allow our
test suite to pass when we haven't yet installed icons system-wide.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
[Pekka: split if-branch into two lines.]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>