smooth bitmap movement with sixel graphics

[dankamongmen]

in #1682 we introduced the pixel-offset fields to ncvisual_options, and added support for Kitty graphics. we ought do the same for Sixel graphics. there we have no in-protocol support; we would need add the necessary transparency rows and columns to the image ourself, with substantially increased processing requirements. this might be post-3.0 material as a result.

[dankamongmen]

make sure we get enough progress on this here and in fbcon that the unit test no longer blows up when run there, as it is now.