graphics/darktable: Update to 2.6.0

Release notes:

• https://www.darktable.org/2018/12/darktable-260-released/

The initial patch was provided by Darren Mulligan. On top of it:

• Warnings about USES=gl and USES=gnome were fixed.
• The CMake CMAKE_POLICY_DEFAULT_CMP0056 setting was removed.

PR: 234800
Submitted by: Darren Mulligan <fixer@bsdmail.com>
Reported by: kunda <chitty_cloud@me.com>