So, I built a project which uses pix_mix, and I couldn't help but notice that the help example doesn't work. Specifically, pix_mix only updates when the left image is updated (unless it works differently on different platforms). So it works fine for movies, but if you have static images and you adjust the mixer input, it doesn't change anything. In the help example, this is exactly what happens. You load two images and then play with the slider and then nothing happens.
In most objects, you could deal with this by just banging the hot input, but banging pix_mix's input just results in an error message that it doesn't know what to do with a bang. In my patch, I've worked around this by storing the left image in a pix_buf and banging the pix_buf, but it doesn't quite seem ideal.