modules/flow: add colors

This is a first stab at colorizing the output.

The flow field now has two v3f_t datums per cell, direction and
color.

It's a bit pastel-y and color choice/palettes definitely needs
work, at least some gamma correction would make sense.

But I kind of like the pastel look actually, some of the
combinations start looking very 80s aesthetic.

A good way to watch flow's possibilities is:

--module=rtv,channels=flow,duration=10,context_duration=10,caption_duration=0 \
--video=sdl,fullscreen=on --defaults --go

The long-ish duration really gives a chance to get into the
groove of things before switching

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