Sixel: emit to the bottom row

As @j4_james says in https://github.com/wez/wezterm/issues/217#issuecomment-1015502154 , it should be possible to write to the bottom row without scrolling.

DECSET 8452 didn't work, but perhaps DECSDM can, thanks to HQSixelEncoder supporting transparency.

For rows on the bottom:

1. Reset DECSDM.
2. Build the image as full window height where all but the bottom cell row, and any cells outside the image cells, are transparent.
3. Emit sixel.
4. Set DECSDM.
5. Don't cache the result.

Try it and see. It might be a lot easier than I thought. (And if it works, support DECSDM in tterminal.ECMA48.) It would be expensive on the TE side as we build basically a full-size screen image nearly all of which is transparent, but ideally said terminal will be removing the transparent cells when it assigns images to the screen.