This fixes an issue where an 'underline' cursor wasn't visible on
underlined text - the cursor was rendered first, and then shadowed by
the text underline.
This fixes an issue where we failed to restore the cursor correctly
when exiting from the alternate screen, if the client had sent escapes
to save the cursor position while inside the alternate screen.
This was because we used the *same* storage for saving the cursor
position through escapes, as for saving it when entering the alternate
screen.
Fix by using a custom variable dedicated to normal <--> alt screen
switching.
Update the sixels' 'row' attribute when re-flowing a grid, to ensure
it is rendered at the correct place.
This should work in most cases, but will break when the cell size has
changed (e.g. font size increase/decrease, or a DPI change).
This patch also moves the sixel image list from the terminal struct
into the grid struct. The sixels are per-grid after all.
