sixel: fix some weird artifacts

Since getColor's color assignment eagerly fills in holes with the
nearest neighbor of the first such observed node, it is prone to falling
into a local minimum where dithering ends up switching between a few
very wrong colors.

Ensure this doesn't happen by always allocating at least 4 nodes of the
octree branch. (Allocating all 7 seems to result in a heavy performance
hit; just 4 means at worst 4096 extra nodes, but likely a lot less, and
I don't see a difference in either output or performance.)

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