sixel: fix row height calculation in resize_vertically()
In resize_vertically(), we assumed a sixel is 6 pixels tall. This is
mostly true, but not for non-1:1 sixels. Or, to be more precise, not
for sixels where 'pan' != 1.
This caused us to allocate too little backing memory, resulting in a
crash when we later tried to write to the image.
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