NEW: graphics/zimg

Hi ports --

Attached is a new port, graphics/zimg. Zimg is a scaling, colorspace
conversion, and dithering library.

---
pkg/DESCR:
The "z" library implements the commonly required image processing basics
of scaling, colorspace conversion, and depth conversion. A simple API
enables conversion between any supported formats to operate with minimal
knowledge from the programmer. All library routines were designed from
the ground-up with correctness, flexibility, and thread-safety as first
priorities. Allocation, buffering, and I/O are cleanly separated from
processing, allowing the programmer to adapt "z" to many scenarios.
---

This is a new LDEP of HandBrake, prompted by Ruben Llorente's HandBrake
update diff.

HandBrake is happy with this on amd64.

OK?

~Brian