Premultiply alpha

void ImgPremultiply( void )
{
    Assert( surface[0].GetNumChannels() == 4 );

    Uint8 *channels = surface[0].GetChannels();
    for ( Uint32 i = 0; i < surface[0].GetPixelCount(); ++i )
    {
        Uint8 *p = &(channels[i*4]);
        p[0] = Uint8(p[0] * (p[3] / 255.0f));
        p[1] = Uint8(p[1] * (p[3] / 255.0f));
        p[2] = Uint8(p[2] * (p[3] / 255.0f));
    }
}