graphics/sekrit-twc-zimg: update to 2.4

Changes: https://github.com/sekrit-twc/zimg/blob/release-2.4/ChangeLog
ABI: https://people.freebsd.org/~jbeich/compat_reports/zimg/2.3_to_2.4/compat_report.html
MFH: 2017Q1