Add support for CYMK JPEGs

[neonichu]

Can be based off this:

template <J_COLOR_SPACE colorSpace>
void setPixel(ImageFrame& buffer, ImageFrame::PixelData* currentAddress, JSAMPARRAY samples, int column)
{
    JSAMPLE* jsample = *samples + column * (colorSpace == JCS_RGB ? 3 : 4);

    switch (colorSpace) {
        case JCS_RGB:
            buffer.setRGBA(currentAddress, jsample[0], jsample[1], jsample[2], 0xFF);
            break;
        case JCS_CMYK:
            // Source is 'Inverted CMYK', output is RGB.
            // See: http://www.easyrgb.com/math.php?MATH=M12#text12
            // Or: http://www.ilkeratalay.com/colorspacesfaq.php#rgb
            // From CMYK to CMY:
            // X =   X    * (1 -   K   ) +   K  [for X = C, M, or Y]
            // Thus, from Inverted CMYK to CMY is:
            // X = (1-iX) * (1 - (1-iK)) + (1-iK) => 1 - iX*iK
            // From CMY (0..1) to RGB (0..1):
            // R = 1 - C => 1 - (1 - iC*iK) => iC*iK  [G and B similar]
            unsigned k = jsample[3];
            buffer.setRGBA(currentAddress, jsample[0] * k / 255, jsample[1] * k / 255, jsample[2] * k / 255, 0xFF);
            break;
    }
}