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.
devc->step is not reset on acquistion start, so acquisition
starts with a different value every time. Thats annoying when
using the demo driver to debug sigrok, so lets make sure that
it's reset to 0.
Signed-off-by: Sven Schnelle <svens@stackframe.org>