demo: add "cable squid" logic waveform (works-with logo, many channels)

demo: add "cable squid" logic waveform (works-with logo, many channels)

Extend the demo driver, add another waveform choice for logic channels.
Create a "cable squid" logo representation which occupies a large number
of channels.

This pattern occupies 128x128 pixels. Unlike the 'sigrok' pattern it
gets repeated when more channels are involved, but is not shifted in the
repetition.