Pixel: add support of pixel formats 4:4:0, 4:1:1 and 4:1:0
Pixel: clean constructor from an AVPixelFormat
Merge pull request #6 from cchampet/dev_add_support_pixelFormats

Supported chroma subsampling :
* 4:4:4
* 4:4:0
* 4:2:2
* 4:2:0
* 4:1:1
* 4:1:0