• Allow to build OpenImageIO without OpenCV, JPEG 2000, and TrueType support
• Turn off FFMPEG option by default; fix spelling of "FFmpeg" and "Color"
• Make sure that `graphics/py-openimageio' slave port remains optionless
• While here, wrap couple of long lines and beautify port description text

OpenCV and FFMPEG are heavy dependencies, while these features might not even
be used in consumer ports (e.g. blender, luxrender, and appleseed). More to
it, OpenCV is broken on PowerPC, which prevents building any consumer ports.

Approved by: maintainer