colorspace conversion and scaling (rave_ops and rave_ops_nvidia)

[gerwin3]

For CPU colorspace conversion and scaling:

• Building it from scratch is very hard though it could be a great opportunity to use Rust SIMD
• ffmpeg has some built-in conversions and scaling: they are pretty good and partially SIMD (but not the fastest possible)
• zimg seems to have most of what is needed so maybe vendor it in?
• Intels IPP has it: https://www.intel.com/content/www/us/en/developer/articles/technical/ipp-calling-image-processing-functions.html (https://github.com/astraw/ipp-sys)