shared: Update all users of DATADIR

shared: Update all users of DATADIR

Replace every use of DATADIR to create a filename with a call to the new
function that allows overriding DATADIR with an env var at runtime.

No attention is paid to asprintf failure.

This restores make distcheck to a passing state after commit 6b58ea
began checking cairo surfaces for validity and exchanged undefined
behaviour we shouldn't have been dependent on for consistent test failure.

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-branches into two lines]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>