add color type conversion

[Its-Just-Nans]

Add a from_pixel() method

[fintelia]

There's already the PixelWithColorType trait that gives access to the ExtendedColorType associated with a pixel. Though looks like we might be missing a way to convert from ExtendedColorType -> ColorType

[Its-Just-Nans]

I've updated the code

[fintelia]

Sorry for the delay. I think this would be better to expose by adding a method on ExtendedColorType that returns an Option<ColorType>.