term: add sixel background color blending

* blend bgcolor with cell color
* do not round up to 6

We don't use the sixel transparency feature because it's useless: sixel
transparency (AFAICT) only lets us respect alpha == 0, but we should
either respect *all* alpha, or no alpha at all.

Since we do color blending, we *do* respect all alpha, but only of
background color.  This sadly means that text under images gets
destroyed, but at least transparent images don't look like garbage.

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