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>