replace imageformats with gamut. remove alpha

[aferust]

• gamut is an up-to-date solution for imageio. For imageio, dcv switched to gamut.
• Computer Vision has nothing to do with alpha channels, so the alpha channel was kept apart from imageio and videoio.
• A small bug in dcv.plot was fixed. It was an issue for some png images.

[9il]

IF_RGB = 3?

[aferust]

IF_RGB enum value doesn't represent channel count. ImageFormat enum value must be a correct index of imageFormatChannelCount. To clarify it I am adding explicit values of enum members so that:

enum ImageFormat
{
    IF_UNASSIGNED = 0, /// Not assigned format.
    IF_MONO = 1, /// Mono, single channel format.
    //IF_MONO_ALPHA, /// Mono with alpha channel.
    IF_RGB = 2, /// RGB format.
    IF_BGR = 3, /// BGR format.
    IF_YUV = 4, /// YUV (YCbCr) format.
    //IF_RGB_ALPHA, /// RGB format with alpha.
    //IF_BGR_ALPHA /// BGR format with alpha.
}

...
immutable size_t[] imageFormatChannelCount = [
    0, // unassigned
    1, // mono
    //2, // mono alpha
    3, // rgb
    3, // bgr
    3, // yuv
    //4, // rgba
    //4 // bgra
    ];

[9il]

ah, got it, thank you