Add rez resizer

As it's not implementing nearest-neighbor, it's disadvantaged compare to some others.
Yet, in my few tests, it seems to be faster than all other pure go solutions.

It doesn't implement resizing rgb or rgb<->yuv, so it may fail on some inputs, but it's on my todo list.

IMHO, you should really separate the jpeg encoding/decoding, if you really want to benchmark resize speeds.