expose pixel offsets in sprixel plane move, somehow

[dankamongmen]

In Kitty, it would be desirable to add the concept of pixel offset within a cell to our idea of moving graphics. Right now, smooth movement requires continuous redrawing, as the ncvisual_options struct is the only place you can express these offsets.

Of course, we don't want these in ncplane_move_yx(). We'd add some function which verifies that it is a sprixel plane, and then calls the internal mover.

This won't be of any benefit on Sixel, where we have to redraw in order to do smooth movement at all. It is thus independent of #2258 , and probably ought be added before 3.0.0 is finalized.

[dankamongmen]

we can add functions whenever we want. even if it doesn't improve sixel, i'd still like it to be usable on sixel before publicizing it (though admittedly we didn't hold off for ncvisual_blit()).