This change has 2 purposes:
  * fix gradient background when writing pdf. Before a gradient filled
frame would be displayed as white (or transparent), because the transp.
rendering mode was taking precedence over the gradient one.
  * making the code more readable by grouping multiple conditions in
an enum.

Change-Id: I5a30756b72be5eabf364c67e175c7b9d22bfa1c0
Reviewed-on: https://gerrit.libreoffice.org/5759Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>