Using PDF's Axial shading leads to better visual result than Sampled
function shading.
Only GradientStyle_LINEAR and GradientStyle_AXIAL are affected by
this change, all other gradient styles are exported as before.

Change-Id: Ib4d549987e34b7ba5d1c5adaf30908f2e306d07e
Reviewed-on: https://gerrit.libreoffice.org/5799Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>