Cover NFX.Graphics with unit tests

[itadapter]

Such as (some rough ideas):

• create various sized images, Image.Of(......)
• Setpixel to preset colors
• Save to file, stream, byte[], read back
• Aver that read back image color are the same as the drawn presets etc...

[sergey-msu]

Image & Canvas test are done. Color extraction tests will be made later