Multi-threaded decoding process

[sincos2854]

By setting the maxThreads variable in the avifDecoder structure, the avifDecoderNextImage function is multithreaded.
By setting the maxThreads variable in the avifRGBImage structure, the avifImageYUVToRGB function is multithreaded.

If this pull request meets with your approval, could you kindly proceed with the build and subsequent release?
Your consideration is greatly appreciated.

[uyjulian]

Thanks! Version 2.0.6 has also been built and released.

[sincos2854]

Thank you very much.