sixel: erase: fix clearing of cell->attrs.clean
When erasing a sixel, the cells underneath it must be marked as
'dirty', in order to be re-rendered.
This was not being done correctly; the for loop loops *from* the start
col, meaning the *end* col is *not* sixel->pos.col, as that's
the *number* of columns, not the *end* column.
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